Semiconductor device and method for manufacturing the semiconductor device

ABSTRACT

A semiconductor device having a high on-state current is provided. 
     The semiconductor device includes a first oxide; a first conductor and a second conductor provided over the first oxide to be separated from each other; and a second oxide provided over the first oxide and between the first conductor and the second conductor. Each of the first oxide and the second oxide has crystallinity, the first oxide includes a region where a c-axis is aligned substantially perpendicularly to a top surface of the first oxide, and the second oxide includes a region where the c-axis is aligned substantially perpendicularly to the top surface of the first oxide, a region where the c-axis is aligned substantially perpendicularly to a side surface of the first conductor, and a region where the c-axis is aligned substantially perpendicularly to a side surface of the second conductor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/766,425, filed May 22, 2020, now allowed, which is incorporated byreference and is a U.S. National Phase Application under 35 U.S.C. § 371of International Application PCT/IB2018/059379, filed on Nov. 28, 2018,which is incorporated by reference and claims the benefit of foreignpriority applications filed in Japan on Dec. 6, 2017, as Application No.2017-234007 and on Mar. 6, 2018, as Application No. 2018-039420.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice and a method for manufacturing the semiconductor device. Anotherembodiment of the present invention relates to a semiconductor wafer, amodule, and an electronic device.

Note that in this specification and the like, a semiconductor devicegenerally means a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach one embodiment of a semiconductor device. It can be sometimes saidthat a display device (a liquid crystal display device, a light-emittingdisplay device, and the like), a projection device, a lighting device,an electro-optical device, a power storage device, a memory device, asemiconductor circuit, an imaging device, an electronic device, and thelike include a semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. One embodiment of the invention disclosed in thisspecification and the like relates to an object, a method, or amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition (composition ofmatter).

BACKGROUND ART

A silicon-based semiconductor material is widely known as asemiconductor thin film that can be used in a transistor, and as anothermaterial, an oxide semiconductor has attracted attention. As the oxidesemiconductor, not only single-component metal oxides, such as indiumoxide and zinc oxide, but also multi-component metal oxides are known.Among the multi-component metal oxides, in particular, an In—Ga—Zn oxide(hereinafter also referred to as IGZO) has been actively studied.

From the studies on IGZO, a CAAC (c-axis aligned crystalline) structureand an nc (nanocrystalline) structure, which are not single crystal noramorphous, have been found in an oxide semiconductor (see Non-PatentDocument 1 to Non-Patent Document 3). In Non-Patent Document 1 andNon-Patent Document 2, a technique for fabricating a transistor using anoxide semiconductor having a CAAC structure is also disclosed. Moreover,Non-Patent Document 4 and Non-Patent Document 5 show that a fine crystalis included even in an oxide semiconductor which has lower crystallinitythan an oxide semiconductor having the CAAC structure or the ncstructure.

In addition, a transistor using IGZO for an active layer has anextremely low off-state current (see Non-Patent Document 6), and an LSIand a display utilizing the characteristics have been reported (seeNon-Patent Document 7 and Non-Patent Document 8).

REFERENCE Non-Patent Document

-   [Non-Patent Document 1] S. Yamazaki et al., “SID Symposium Digest of    Technical Papers”, 2012, volume 43, issue 1, pp. 183-186.-   [Non-Patent Document 2] S. Yamazaki et al., “Japanese Journal of    Applied Physics”, 2014, volume 53, Number 4S, pp.    04ED18-1-04ED18-10.-   [Non-Patent Document 3] S. Ito et al., “The Proceedings of AM-FPD'13    Digest of Technical Papers”, 2013, pp. 151-154.-   [Non-Patent Document 4] S. Yamazaki et al., “ECS Journal of Solid    State Science and Technology”, 2014, volume 3, issue 9, pp.    Q3012-Q3022.-   [Non-Patent Document 5] S. Yamazaki, “ECS Transactions”, 2014,    volume 64, issue 10, pp. 155-164.-   [Non-Patent Document 6] K. Kato et al., “Japanese Journal of Applied    Physics”, 2012, volume 51, pp. 021201-1-021201-7.-   [Non-Patent Document 7] S. Matsuda et al., “2015 Symposium on VLSI    Technology Digest of Technical Papers”, 2015, pp. T216-T217.-   [Non-Patent Document 8] S. Amano et al., “SID Symposium Digest of    Technical Papers”, 2010, volume 41, issue 1, pp. 626-629.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of one embodiment of the present invention is to provide asemiconductor device having a high on-state current. Another object ofone embodiment of the present invention is to provide a semiconductordevice having high frequency characteristics. Another object of oneembodiment of the present invention is to provide a semiconductor devicewith favorable reliability. Another object of one embodiment of thepresent invention is to provide a semiconductor device that can beminiaturized or highly integrated. Another object of one embodiment ofthe present invention is to provide a semiconductor device havingfavorable electrical characteristics. Another object of one embodimentof the present invention is to provide a semiconductor device with highproductivity.

Another object of one embodiment of the present invention is to providea semiconductor device that can retain data for a long time. Anotherobject of one embodiment of the present invention is to provide asemiconductor device capable of high-speed data writing. Another objectof one embodiment of the present invention is to provide a semiconductordevice with high design flexibility. Another object of one embodiment ofthe present invention is to provide a semiconductor device in whichpower consumption can be reduced. Another object of one embodiment ofthe present invention is to provide a novel semiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. One embodiment of the present invention does not haveto achieve all the objects. Other objects will be apparent from and canbe derived from the description of the specification, the drawings, theclaims, and the like.

Means for Solving the Problems

One embodiment of the present invention is a semiconductor deviceincluding a first oxide; a first conductor and a second conductorprovided over the first oxide to be separated from each other; and asecond oxide provided over the first oxide and between the firstconductor and the second conductor, in which each of the first oxide andthe second oxide has crystallinity, the first oxide includes a regionwhere a c-axis is aligned substantially perpendicularly to a top surfaceof the first oxide, and the second oxide includes a region where thec-axis is aligned substantially perpendicularly to the top surface ofthe first oxide, a region where the c-axis is aligned substantiallyperpendicularly to a side surface of the first conductor, and a regionwhere the c-axis is aligned substantially perpendicularly to a sidesurface of the second conductor.

Another embodiment of the present invention is a semiconductor deviceincluding a first oxide; a first conductor and a second conductorprovided over the first oxide to be separated from each other; a firstinsulator which is provided over the first conductor and the secondconductor and in which an opening is formed to overlap with a regionbetween the first conductor and the second conductor; a third conductorprovided in the opening; a second insulator provided between the thirdconductor and the first oxide, the first conductor, the secondconductor, and the first insulator; and a second oxide provided betweenthe second insulator and the first oxide, the first conductor, thesecond conductor, and the first insulator, in which each of the firstoxide and the second oxide has crystallinity, the first oxide includes aregion where a c-axis is aligned substantially perpendicularly to a topsurface of the first oxide, and the second oxide includes a region wherethe c-axis is aligned substantially perpendicularly to the top surfaceof the first oxide, a region where the c-axis is aligned substantiallyperpendicularly to a side surface of the first conductor, and a regionwhere the c-axis is aligned substantially perpendicularly to a sidesurface of the second conductor.

In the above embodiment, a third insulator may be provided between thefirst insulator and the first oxide, the first conductor, the secondconductor, and the second oxide, and the third insulator may have alower oxygen permeability than the first insulator.

In the above embodiment, the third insulator may be provided between thefirst insulator and the first oxide, the first conductor, and the secondconductor, the third insulator may have a lower oxygen permeability thanthe first insulator, and the second oxide may be in contact with thefirst insulator.

In the above embodiments, the third conductor may be provided betweenthe first conductor and the first oxide, a fourth conductor may beprovided between the second conductor and the first oxide, part of thefirst conductor may be in contact with the top surface of the firstoxide, and part of the second conductor may be in contact with the topsurface of the first oxide. In the above embodiment, the distancebetween the first conductor and the second conductor may be shorter thanthe length of the opening in the channel length direction.

In the above embodiments, the side surface of the first conductor may besubstantially perpendicular to a bottom surface of the first conductor,and the side surface of the second conductor may be substantiallyperpendicular to a bottom surface of the second conductor.

In the above embodiments, the angle formed by the side surface of thefirst conductor and the bottom surface of the first conductor may begreater than or equal to 100 and less than or equal to 80°, and theangle formed by the side surface of the second conductor and the bottomsurface of the second conductor may be greater than or equal to 100 andless than or equal to 80°.

In any of the above embodiments, the first oxide and the second oxidemay contain In, an element M (M is Al, Ga, Y, or Sn), and Zn.Furthermore, in the above embodiment, the atomic ratio of In to theelement M in the first oxide may be higher than the atomic ratio of Into the element M in the second oxide.

In the above embodiments, the c-axis in the first oxide alignedsubstantially perpendicularly to the top surface of the first oxide andthe c-axis in the second oxide aligned substantially perpendicularly tothe top surface of the first oxide may be substantially continuous.

In the above embodiment, a third oxide may be provided under the firstoxide. In the above embodiment, the energy of the conduction bandminimum of each of the second oxide and the third oxide may be higherthan the energy of the conduction band minimum of the first oxide. Inany of the above embodiments, the first oxide, the second oxide, and thethird oxide may contain In, the element M (M is Al, Ga, Y, or Sn), andZn. Furthermore, in the above embodiment, the atomic ratio of In to theelement M in the first oxide may be higher than the atomic ratio of Into the element M in each of the second oxide and the third oxide.

Effect of the Invention

According to one embodiment of the present invention, a semiconductordevice having a high on-state current can be provided. According to oneembodiment of the present invention, a semiconductor device having highfrequency characteristics can be provided. According to one embodimentof the present invention, a semiconductor device with favorablereliability can be provided. According to one embodiment of the presentinvention, a semiconductor device that can be miniaturized or highlyintegrated can be provided. According to one embodiment of the presentinvention, a semiconductor device having favorable electricalcharacteristics can be provided. According to one embodiment of thepresent invention, a semiconductor device with high productivity can beprovided.

A semiconductor device that can retain data for a long time can beprovided. A semiconductor device capable of high-speed data writing canbe provided. A semiconductor device with high design flexibility can beprovided. A semiconductor device in which power consumption can bereduced can be provided. A novel semiconductor device can be provided.

Note that the descriptions of the effects do not disturb the existenceof other effects. One embodiment of the present invention does not haveto have all of these effects. Effects other than these will be apparentfrom the description of the specification, the drawings, the claims, andthe like and effects other than these can be derived from thedescription of the specification, the drawings, the claims, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C Cross-sectional views of semiconductor devices ofembodiments of the present invention.

FIG. 2 A diagram illustrating a movement path of oxygen in an In—Ga—Znoxide.

FIGS. 3A to 3C A top view and cross-sectional views of a semiconductordevice of one embodiment of the present invention.

FIGS. 4A to 4C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 5A to 5C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 6A to 6C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 7A to 7C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 8A to 8C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 9A to 9C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 10A to 10C A top view and cross-sectional views of a semiconductordevice of one embodiment of the present invention.

FIGS. 11A to 11C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 12A to 12C A top view and cross-sectional views of a semiconductordevice of one embodiment of the present invention.

FIGS. 13A to 13C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 14A to 14C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 15A to 15C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 16A to 16C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 17A to 17C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 18A to 18C A top view and cross-sectional views of a semiconductordevice of one embodiment of the present invention.

FIGS. 19A to 19C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIGS. 20A to 20C A top view and cross-sectional views illustrating amethod for manufacturing a semiconductor device of one embodiment of thepresent invention.

FIG. 21 A cross-sectional view illustrating a structure of a memorydevice of one embodiment of the present invention.

FIG. 22 A cross-sectional view illustrating a structure of a memorydevice of one embodiment of the present invention.

FIGS. 23A and 23B A block diagram illustrating a structure example of amemory device of one embodiment of the present invention.

FIGS. 24A to 24H Circuit diagrams each illustrating a structure exampleof a memory device of one embodiment of the present invention.

FIGS. 25A and 25B Schematic views of a semiconductor device of oneembodiment of the present invention.

FIGS. 26A to 26E Schematic views of memory devices of one embodiment ofthe present invention.

FIGS. 27A, 27B, 27C, 27D, 27E1, 27E2, and 27F Diagrams illustratingelectronic devices of one embodiment of the present invention.

FIG. 28 A cross-sectional TEM image of Example of the present invention.

FIGS. 29A to 29E Cross-sectional TEM images of Example of the presentinvention.

FIG. 30 A cross-sectional TEM image of Example of the present invention.

FIGS. 31A and 31B A cross-sectional TEM image and a crystal orientationmap of Example of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to thedrawings. Note that the embodiments can be implemented with manydifferent modes, and it will be readily appreciated by those skilled inthe art that modes and details thereof can be changed in various wayswithout departing from the spirit and scope thereof. Therefore, thepresent invention should not be interpreted as being limited to thedescription of the embodiments below.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, they are not limitedto the illustrated scale. Note that the drawings are schematic viewsshowing ideal examples, and shapes or values are not limited to thoseshown in the drawings. For example, in the actual manufacturing process,a layer, a resist mask, or the like might be unintentionally reduced insize by treatment such as etching, which might not be reflected in thedrawings for easy understanding. In the drawings, the same referencenumerals are used for the same portions or portions having similarfunctions in different drawings, and repeated description thereof isomitted in some cases. Furthermore, the same hatch pattern is used forthe portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

Particularly in a top view (also referred to as a plan view), aperspective view, or the like, the description of some components mightbe omitted for easy understanding of the invention. Furthermore, thedescription of some hidden lines and the like might be omitted.

In this specification and the like, ordinal numbers such as first andsecond are used for convenience and do not denote the order of steps orthe stacking order of layers. Therefore, for example, description can bemade when “first” is replaced with “second”, “third”, or the like asappropriate. In addition, the ordinal numbers in this specification andthe like are not necessarily the same as the ordinal numbers used tospecify one embodiment of the present invention.

In this specification and the like, the terms for explainingarrangement, such as “over” and “under”, are used for convenience todescribe the positional relation between components with reference todrawings. The positional relation between components is changed asappropriate in accordance with a direction in which the components aredescribed. Thus, terms for the description are not limited to those usedin this specification, and description can be made appropriatelydepending on the situation.

In the case where there is an explicit description, X and Y areconnected, in this specification and the like, for example, the casewhere X and Y are electrically connected, the case where X and Y arefunctionally connected, and the case where X and Y are directlyconnected are disclosed in this specification and the like. Accordingly,without being limited to a predetermined connection relation, forexample, a connection relation shown in drawings or texts, a connectionrelation other than one shown in drawings or texts is regarded as beingdescribed in the drawings or the texts.

Here, X and Y denote an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, or a layer).

Furthermore, functions of a source and a drain might be switched when atransistor of opposite polarity is employed or a direction of currentflow is changed in circuit operation, for example. Therefore, the terms“source” and “drain” can sometimes be used interchangeably in thisspecification and the like.

Note that in this specification and the like, depending on transistorstructures, a channel width in a region where a channel is actuallyformed (hereinafter, referred to as an “effective channel width”) isdifferent from a channel width shown in a top view of a transistor(hereinafter, referred to as an “apparent channel width”) in some cases.For example, when a gate electrode covers a side surface of asemiconductor, an effective channel width is greater than an apparentchannel width, and its influence cannot be ignored in some cases. Forexample, in a miniaturized transistor having a gate electrode covering aside surface of a semiconductor, the proportion of a channel formationregion formed in the side surface of the semiconductor is increased insome cases. In that case, an effective channel width is greater than anapparent channel width.

In such a case, an effective channel width is difficult to estimate byactual measurement in some cases. For example, to estimate an effectivechannel width from a design value, it is necessary to assume that theshape of a semiconductor is known. Therefore, in the case where theshape of a semiconductor is not known accurately, it is difficult tomeasure an effective channel width accurately.

In this specification, the simple term “channel width” refers to anapparent channel width in some cases. Alternatively, in thisspecification, the simple term “channel width” refers to an effectivechannel width in some cases. Note that values of a channel length, achannel width, an effective channel width, an apparent channel width,and the like can be determined, for example, by analyzing across-sectional TEM image and the like.

Note that impurities in a semiconductor refer to, for example, elementsother than the main components of a semiconductor. For example, anelement with a concentration of lower than 0.1 atomic % can be regardedas an impurity. When an impurity is contained, for example, DOS (Densityof States) in a semiconductor may be increased or the crystallinity maybe decreased. In the case where the semiconductor is an oxidesemiconductor, examples of an impurity that changes characteristics ofthe semiconductor include Group 1 elements, Group 2 elements, Group 13elements, Group 14 elements, Group 15 elements, and transition metalsother than the main components of the oxide semiconductor; hydrogen,lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen aregiven as examples. In the case of an oxide semiconductor, water alsofunctions as an impurity in some cases. In addition, in the case of anoxide semiconductor, oxygen vacancies may be formed by entry ofimpurities, for example. Furthermore, when the semiconductor is silicon,examples of an impurity that changes the characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Note that in this specification and the like, silicon oxynitride is amaterial that contains more oxygen than nitrogen in its composition.Moreover, silicon nitride oxide is a material that contains morenitrogen than oxygen in its composition.

In addition, in this specification and the like, the term “insulator”can be replaced with insulating film or insulating layer. Moreover, theterm “conductor” can be replaced with conductive film or conductivelayer. Furthermore, the term “semiconductor” can be replaced withsemiconductor film or semiconductor layer.

In this specification and the like, “parallel” indicates a state wheretwo straight lines are placed at an angle of greater than or equal to−10° and less than or equal to 10°. Accordingly, the case where theangle is greater than or equal to −5° and less than or equal to 5° isalso included. Moreover, “substantially parallel” indicates a statewhere two straight lines are placed at an angle of greater than or equalto −30° and less than or equal to 30°. In addition, “perpendicular”indicates a state where two straight lines are placed at an angle ofgreater than or equal to 80° and less than or equal to 100°.Accordingly, the case where the angle is greater than or equal to 85°and less than or equal to 95° is also included. Moreover, “substantiallyperpendicular” indicates a state where two straight lines are placed atan angle of greater than or equal to 60° and less than or equal to 120°.

Note that in this specification, a barrier film means a film having afunction of inhibiting the transmission of oxygen and impurities such aswater and hydrogen, and the barrier film having conductivity issometimes referred to as a conductive barrier film.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, in the case where a metal oxide is used in a semiconductorlayer of a transistor, the metal oxide is referred to as an oxidesemiconductor in some cases. That is, an OS FET or an OS transistor canalso be referred to as a transistor including an oxide or an oxidesemiconductor.

In this specification and the like, “normally off” means that a currentper micrometer of channel width flowing through a transistor when apotential is not applied to a gate or a ground potential is applied tothe gate is 1×10⁻²⁰ A or lower at room temperature, 1×10⁻¹⁸ A or lowerat 85° C., or 1×10⁻¹⁶ A or lower at 125° C.

Embodiment 1

The structure and characteristics of a semiconductor device of oneembodiment of the present invention will be described below.

FIG. 1(A) is a cross-sectional view of part of a transistor 10 a of oneembodiment of the present invention.

As illustrated in FIG. 1(A), the transistor 10 a includes an oxide 12provided over a substrate; a conductor 14 a and a conductor 14 bprovided over the oxide 12 to be separated from each other; and an oxide13 provided over the oxide 12 and between the conductor 14 a and theconductor 14 b. Here, each of the conductor 14 a and the conductor 14 bfunctions as a source electrode and a drain electrode of the transistor10 a. Although not illustrated, a gate insulator and a gate electrode ofthe transistor 10 a are provided over the oxide 13 and between theconductor 14 a and the conductor 14 b.

The oxide 12 includes a channel formation region in a region between theconductor 14 a and the conductor 14 b and includes a source region and adrain region in the vicinity of a region overlapping with the conductor14 a (the conductor 14 b) so that the channel formation region issandwiched between the source region and the drain region. Note that thesource region and/or the drain region may have a shape in which thesource region and/or the drain region extends inward from the conductor14 a (the conductor 14 b). Note that the channel formation region of thetransistor 10 a is formed not only in the oxide 12 but also in thevicinity of the interface between the oxide 12 and the oxide 13 and/orin the oxide 13, in some cases.

In the transistor 10 a, a metal oxide functioning as an oxidesemiconductor (hereinafter also referred to as an oxide semiconductor)is preferably used as the oxide 12 and the oxide 13. For example, ametal oxide having an energy gap of 2 eV or more, preferably 2.5 eV ormore is preferably used as a metal oxide to be the oxide 12 and theoxide 13. The off-state current (leakage current) of a transistorincluding a metal oxide having a wide energy gap as described above issmall. When such a transistor is used, a semiconductor device with lowpower consumption can be provided.

For the oxide 12 and the oxide 13, a metal oxide such as an In-M-Znoxide (the element M is one or more kinds selected from aluminum,gallium, yttrium, tin, copper, vanadium, beryllium, boron, titanium,iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium,neodymium, hafnium, tantalum, tungsten, magnesium, and the like) ispreferably used. In particular, aluminum, gallium, yttrium, or tin ispreferably used as the element M. Furthermore, as the oxide 12 and theoxide 13, an In—Ga oxide or an In—Zn oxide may be used.

Here, the atomic ratio of In to the element M in the metal oxide used asthe oxide 12 is preferably higher than the atomic ratio of In to theelement Min the metal oxide used as the oxide 13. When the oxide 13 isprovided over the oxide 12 as described above, impurities can beinhibited from being diffused into the oxide 12 from components formedabove the oxide 13. Furthermore, when the oxide 12 and the oxide 13contain a common element (as its main component) besides oxygen, thedensity of defect states at the interface between the oxide 12 and theoxide 13 can be low. In that case, a main carrier path is the oxide 12or the vicinity thereof, for example, the interface between the oxide 12and the oxide 13. Since the density of defect states at the interfacebetween the oxide 12 and the oxide 13 can be low, the influence ofinterface scattering on carrier conduction is small, and a high on-statecurrent can be obtained.

A transistor using an oxide semiconductor is likely to have itselectrical characteristics changed when impurities and oxygen vacanciesexist in a region of the oxide semiconductor where a channel is formed,which may affect the reliability. Moreover, if the region of the oxidesemiconductor where a channel is formed includes oxygen vacancies, thetransistor tends to have normally-on characteristics. Thus, oxygenvacancies in the region where a channel is formed are preferably reducedas much as possible. Thus, a transistor which has reduced variation inelectrical characteristics, stable electrical characteristics, andimproved reliability can be provided.

Each of the oxide 12 and the oxide 13 preferably has crystallinity. Itis particularly preferable to use a CAAC-OS (c-axis aligned crystallineoxide semiconductor) for the oxide 12 and the oxide 13.

The CAAC-OS has c-axis alignment, a plurality of nanocrystals areconnected in the a-b plane direction, and its crystal structure hasdistortion. Note that the distortion refers to a portion where thedirection of a lattice arrangement changes between a region with aregular lattice arrangement and another region with a regular latticearrangement in a region where the plurality of nanocrystals areconnected.

The nanocrystal is basically a hexagon but is not always a regularhexagon and is a non-regular hexagon in some cases. Furthermore, apentagonal or heptagonal lattice arrangement, for example, is includedin the distortion in some cases. Note that it is difficult to observe aclear crystal grain boundary (also referred to as grain boundary) evenin the vicinity of distortion in the CAAC-OS. That is, formation of acrystal grain boundary is inhibited by the distortion of a latticearrangement. This is because the CAAC-OS can tolerate distortion owingto a low density of arrangement of oxygen atoms in the a-b planedirection, an interatomic bond length changed by substitution of a metalelement, and the like.

The CAAC-OS tends to have a layered crystal structure (also referred toas layered structure) in which a layer containing indium and oxygen(hereinafter, In layer) and a layer containing the element M, zinc, andoxygen (hereinafter, (M,Zn) layer) are stacked. Note that indium and theelement M can be replaced with each other, and when the element M in the(M,Zn) layer is replaced with indium, the layer can also be referred toas an (In, M, Zn) layer. Furthermore, when indium in the In layer isreplaced with the element M, the layer can be referred to as an (In,M)layer.

The CAAC-OS is a metal oxide with high crystallinity. By contrast, inthe CAAC-OS, a reduction in electron mobility due to the crystal grainboundary is less likely to occur because it is difficult to observe aclear crystal grain boundary. Entry of impurities, formation of defects,or the like might decrease the crystallinity of a metal oxide; thus, itcan be said that the CAAC-OS is a metal oxide that has small amounts ofimpurities and defects (e.g., oxygen vacancies (also referred to asVo)). Thus, a metal oxide including a CAAC-OS is physically stable.Therefore, the metal oxide including a CAAC-OS is resistant to heat andhas high reliability.

Here, an example of a CAAC-OS analyzed by X-ray diffraction (XRD) willbe described. For example, when the structure of a CAAC-OS including anInGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appearsat a diffraction angle (2θ) in the neighborhood of 31° in some cases.This peak is assigned to the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS have c-axis alignment, and thatthe c-axes face a direction substantially perpendicular to the formationsurface or the top surface.

Furthermore, an example of a CAAC-OS analyzed by electron diffractionwill be described. For example, when an electron beam with a probediameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄crystal in a direction parallel to the sample surface, a diffractionpattern (also referred to as a selected-area transmission electrondiffraction pattern) appears in some cases. This diffraction patternincludes spots derived from the (009) plane of the InGaZnO₄ crystal.Thus, the electron diffraction also indicates that crystals included inthe CAAC-OS have c-axis alignment, and that the c-axes face a directionsubstantially perpendicular to the formation surface or the top surface.Meanwhile, a ring-like diffraction pattern is shown when an electronbeam with a probe diameter of 300 nm is incident on the same sample in adirection perpendicular to the sample surface. Thus, the electrondiffraction also indicates that the a-axes and b-axes of the crystalsincluded in the CAAC-OS do not have regular alignment.

As described above, the oxide 12 includes a crystal region including alayer 12 a of crystals that extend in the a-b plane direction and ac-axis 12 b perpendicular to the a-b plane direction. Here, in the oxide12, the c-axis 12 b preferably faces a direction substantiallyperpendicular to a formation surface or a top surface of the oxide 12.Thus, as illustrated in FIG. 1 , the oxide 12 includes a region wherethe c-axis 12 b is aligned substantially perpendicularly to the topsurface of the oxide 12.

Furthermore, the oxide 13 includes a crystal region including a layer 13a of crystals that extend in the a-b plane direction and a c-axis 13 bperpendicular to the a-b plane direction. Here, in the oxide 13, thec-axis 13 b preferably faces a direction substantially perpendicular toa formation surface or a top surface of the oxide 13. Thus, asillustrated in FIG. 1 , the oxide 13 includes a region where the c-axis13 b is aligned substantially perpendicularly to the top surface of theoxide 12, a region where the c-axis 13 b is aligned substantiallyperpendicularly to a side surface of the conductor 14 a, and a regionwhere the c-axis 13 b is aligned substantially perpendicularly to a sidesurface of the conductor 14 b.

Although described in detail later, the CAAC-OS has a property in whichoxygen is likely to be transferred in the c-axis direction. Thus, whenthe oxide 13 includes the region where the c-axis 13 b is alignedsubstantially perpendicularly to the side surface of the conductor 14 aand the region where the c-axis 13 b is aligned substantiallyperpendicularly to the side surface of the conductor 14 b as describedabove, excess oxygen from an insulator provided over the oxide 13 andcontaining oxygen (e.g., a gate insulator) can be inhibited from beingabsorbed by the conductor 14 a and the conductor 14 b. Thus, oxidationof the vicinity of the side surfaces of the conductor 14 a and theconductor 14 b that face each other can be inhibited. Thus, the channellength of the transistor 10 a can be prevented from being shorter thanthe distance in design of the conductor 14 a and the conductor 14 b;therefore, the on-state current, the S value, and the frequencycharacteristic can be prevented from being lower than the design values.

Here, the thickness of the oxide 13 is 2 nm or more, preferably 3 nm ormore, further preferably 5 nm or more, for example. The thickness of theoxide 13 is increased as described above, so that the region where thec-axis 13 b is aligned substantially perpendicularly to the side surfaceof the conductor 14 a (the conductor 14 b) can be formed.

When excess oxygen is supplied to an oxide semiconductor, for example,the structure of the excess oxygen in the oxide semiconductor may bechanged by a stress such as a voltage or a high temperature. Thus, thetransistor including the oxide semiconductor might have unstableelectrical characteristics or lower reliability. However, the oxide 13includes the region where the c-axis 13 b is aligned substantiallyperpendicularly to the top surface of the oxide 12 as described above,whereby excess oxygen from an insulator provided over the oxide 13 andcontaining oxygen (e.g., a gate insulator) can be inhibited from beingabsorbed by the oxide 12. Thus, the transistor can have stableelectrical characteristics and increased reliability.

The crystal structures of the oxide 12 and the oxide 13 can be confirmedby taking a cross-sectional TEM image, for example. In addition, a mapobtained by overlapping data on the degree of crystallization and thealignment for each pixel in the cross-sectional TEM image with thecross-sectional TEM image (hereinafter referred to as a crystalorientation map) may be made to perform analysis.

The crystal orientation map can be made as follows. First, a region tobe subjected to FFT (Fast Fourier Transform) using a pixel as a center(hereinafter referred to as a FFT window) is set for each pixel in thecross-sectional TEM image. The FFT window may be set in accordance withthe resolution of the cross-sectional TEM image and the size of a targetcrystal to be observed as appropriate. For example, in the case of theCAAC-OS, the FFT window may have a circular shape with a diameter ofapproximately 1.5 nm on the scale of the cross-sectional TEM image.

Next, each FFT window is subjected to FFT processing to obtain a FFTimage corresponding to each FFT window. In the FFT image, spotsreflecting the crystal structure of the FFT window appear. That is, theintensity of the spots represents the degree of crystallization in theFFT window, and the arrangement of the spots represents the crystalorientation in the FFT window. For example, in the case of thecross-sectional TEM image of the CAAC-OS taken from the directionperpendicular to the c-axis, two spots having high intensity areobserved in the FFT image in some cases. The intensity of the two spotsrepresents the degree of crystallization of the CAAC-OS, and the angleof a line segment obtained by connecting the two spots represents thecrystal orientation of the CAAC-OS.

Next, the pixel of the cross-sectional TEM image is colored inaccordance with the angle of the spots of the FFT image. At this time, apixel having a high spot intensity is colored with low transparency, anda pixel having a low spot intensity is colored with high transparency.In the crystal orientation map made in this manner, the cross-sectionalTEM of a region where the degree of crystallization is low is seen as itis, and a color corresponding to the crystal orientation is seen in aregion where the degree of crystallization is high.

Furthermore, as described above, the oxide 12 includes the region wherethe c-axis 12 b is aligned substantially perpendicularly to the topsurface of the oxide 12, whereby oxygen vacancies, impurities, or thelike in the oxide can be reduced, and the on-state current, the S value,and the frequency characteristic of the transistor 10 a can be improved.Here, the region where the c-axis 12 b is aligned substantiallyperpendicularly to the top surface of the oxide 12 is formed at least inthe channel formation region of the transistor 10 a. That is, in aregion in the vicinity of a source region or a drain region of thetransistor 10 a, i.e., in a region of the oxide 12 in the vicinity ofthe conductor 14 a (the conductor 14 b), the region where the c-axis 12b is aligned substantially perpendicularly to the top surface of theoxide 12 is not formed in some cases.

It is preferable to make the planarity of the formation surface of theoxide 12 favorable in order to provide such an oxide 12. For example,the average surface roughness (Ra) of the formation surface of the oxide12 is less than or equal to 1 nm, preferably less than or equal to 0.5nm, further preferably less than or equal to 0.3 nm.

Note that in this specification and the like, average surface roughness(Ra) is obtained by three-dimensional expansion of arithmetic meansurface roughness that is defined by JIS B 0601:2001 (IS04287:1997) soas to be applied to a curved surface, and is represented by an averagevalue of the absolute values of deviations from a reference surface to aspecific surface. The average surface roughness (Ra) can be measuredusing an atomic force microscope (AFM).

Moreover, it is preferable that the c-axis 12 b in the oxide 12 that isaligned substantially perpendicularly to the top surface of the oxide 12and the c-axis 13 b in the oxide 13 that is aligned substantiallyperpendicularly to the top surface of the oxide 12 be substantiallycontinuous. Here, “the c-axis of the oxide 12 and the c-axis of theoxide 13 are substantially continuous” refers to a state in which aclear crystal grain boundary cannot be observed in a cross-sectional TEMimage of a stacked film of the oxide 12 and the oxide 13, for example.By formation of such an oxide 12 and such an oxide 13, the density ofdefect states at the interface between the oxide 12 and the oxide 13 canbe decreased, and thus the influence of interface scattering on carrierconduction becomes small, and a high on-state current can be obtained.

In order to provide the oxide 12 and the oxide 13 in this manner, it ispreferable that the oxide 12 be deposited over the formation surfacehaving high planarity and that the oxide 13 be deposited in a statewhere impurities and the like over the oxide 12 are removed as much aspossible, as described above.

In the transistor 10 a, the side surface of the conductor 14 a issubstantially perpendicular to a bottom surface of the conductor 14 a,and the side surface of the conductor 14 b is substantiallyperpendicular to a bottom surface of the conductor 14 b. However, thepresent application is not limited thereto. As in a transistor 10 billustrated in FIG. 1(B), side surfaces of the conductor 14 a and theconductor 14 b that face each other may have a tapered shape. Here, anangle (θ) formed between the side surface of the conductor 14 a and thebottom surface of the conductor 14 a is greater than or equal to 100 andless than or equal to 80°, preferably greater than or equal to 300 andless than or equal to 60°. An angle (θ) formed between the side surfaceof the conductor 14 b and the bottom surface of the conductor 14 b isgreater than or equal to 10° and less than or equal to 80°, preferablygreater than or equal to 30° and less than or equal to 60°. Thus, anelectric field of a gate electrode largely contributes also to theregion of the oxide 12 in the vicinity of the conductor 14 a and theconductor 14 b, so that the on-state current, the S value, and thefrequency characteristic of the transistor 10 b can be improved.

Also in the case where the conductor 14 a and the conductor 14 b have atapered shape as described above, as illustrated in FIG. 1(B), the oxide13 includes the region where the c-axis 13 b is aligned substantiallyperpendicularly to the side surface of the conductor 14 a and the regionwhere the c-axis 13 b is aligned substantially perpendicularly to theside surface of the conductor 14 b.

As in a transistor 10 c illustrated in FIG. 1(C), side surfaces of theconductor 14 a and the conductor 14 b that face each other may have aplurality of surfaces. Angles (θ₁, θ₂) formed by the plurality ofsurfaces and the bottom surface of the conductor 14 a (the conductor 14b) are different from each other. The oxide 13 includes a region wherethe c-axis 13 b is aligned substantially perpendicularly to each of theplurality of surfaces. Note that although the side surface of theconductor 14 a (the conductor 14 b) has only the surface where the angleformed by the side surface and the bottom surface is θ₁ and the surfacewhere the angle formed by the side surface and the bottom surface is θ₂in FIG. 1(C), the semiconductor device in this embodiment is not limitedto this, and the side surface of the conductor 14 a (the conductor 14 b)may have more surfaces.

According to one embodiment of the present invention, a semiconductordevice having a high on-state current can be provided. According to oneembodiment of the present invention, a semiconductor device having highfrequency characteristics can be provided. According to one embodimentof the present invention, a semiconductor device with favorablereliability can be provided. According to one embodiment of the presentinvention, a semiconductor device having favorable electricalcharacteristics can be provided. According to one embodiment of thepresent invention, a semiconductor device having a low off-state currentcan be provided. According to one embodiment of the present invention, asemiconductor device with low power consumption can be provided.

<Movement of Oxygen in In—Ga—Zn Oxide>

A relationship between crystallinity and oxygen permeability in the casewhere the oxide 13 is an In—Ga—Zn oxide is described below.

An energy barrier due to movement of excess oxygen (oxygen) in a crystalof an In—Ga—Zn oxide is obtained by calculation. In the calculation,plane-wave basis first-principles calculation software VASP (Viennaab-initio simulation package) based on density functional theory isused. Note that GGA-PBE is used as a functional. Cut-off energy of aplane wave is 400 eV. The effect of an inner shell electron is includedby a PAW (Projector Augmented Wave) method.

Here, the ease of movement of excess oxygen (oxygen) through a movementpath 1, a movement path 2, a movement path 3, and a movement path 4 in acrystal of an In—Ga—Zn oxide illustrated in FIG. 2 is calculated. Notethat the a-axis, the b-axis, and the c-axis in FIG. 2 correspond to acrystal structure of an In—Ga—Zn oxide.

Note that the movement path 1 is a path through which excess oxygen(oxygen) bonded to oxygen bonded to three indium atoms and one zinc atomis bonded to adjacent oxygen bonded to three indium atoms and one zincatom. The movement path 2 is a path through which excess oxygen (oxygen)bonded to oxygen bonded to three indium atoms and one gallium atomcrosses a layer containing indium and oxygen and is bonded to adjacentoxygen bonded to three indium atoms and one zinc atom. The movement path3 is a path through which excess oxygen (oxygen) bonded to oxygen bondedto two gallium atoms and one zinc atom is bonded to adjacent oxygenbonded to two zinc atoms and one gallium atom. The movement path 4 is apath through which excess oxygen (oxygen) bonded to oxygen bonded to twogallium atoms and one zinc atom crosses a layer containing gallium,zinc, and oxygen and is bonded to adjacent oxygen bonded to three indiumatoms and one gallium atom.

When the frequency of going over an energy barrier E_(a) of diffusionper unit time is referred to as a diffusion frequency R, R can beexpressed as the following formula.R=v·exp[−E _(a)/(k _(B) T)]

Note that v represents the number of heat vibrations of diffusion atoms,k_(B) represents Boltzmann constant, and T represents the absolutetemperature. The diffusion frequency R at 350° C. and 450° C. when 10¹³[1/sec] is applied to v as Debye frequency is shown in Table 1.

TABLE 1 Energy barrier Diffusion frequency R [1/sec] [eV] 350° C. 450°C. Movement path 1 0.50 9.0 × 10⁸ 3.3 × 10⁹ Movement path 2 1.97  1.2 ×10⁻³  1.9 × 10⁻¹ Movement path 3 0.53 5.2 × 10⁸ 2.0 × 10⁹ Movement path4 0.56 3.0 × 10⁸ 1.3 × 10⁹

As shown in Table 1, it is found that the movement path 2 across thelayer containing indium and oxygen has a higher energy barrier than theother movement paths. This indicates that movement of excess oxygen(oxygen) in the c-axis direction is less likely to occur in a crystal ofan In—Ga—Zn oxide. In other words, in the case where crystals havec-axis alignment and the c-axes are aligned in a direction substantiallyperpendicular to a formation surface or a top surface, like CAAC-OS,movement of excess oxygen (oxygen) is less likely to occur in thedirection substantially perpendicular to the formation surface or thetop surface.

The structure, method, and the like described above in this embodimentcan be used in combination as appropriate with the structures, methods,and the like described in the other embodiments.

Embodiment 2

Specific structure examples of the semiconductor device described in theabove embodiment will be described below with reference to FIG. 3 toFIG. 20 .

<Structure Example of Semiconductor Device>

FIG. 3(A), FIG. 3(B), and FIG. 3(C) are a top view and cross-sectionalviews of a transistor 200 of one embodiment of the present invention anda periphery of the transistor 200.

FIG. 3(A) is a top view of a semiconductor device including thetransistor 200. FIG. 3(B) and FIG. 3(C) are cross-sectional views of thesemiconductor device. Here, FIG. 3(B) is a cross-sectional view of aportion indicated by a dashed-dotted line A1-A2 in FIG. 3(A), and is across-sectional view in the channel length direction of the transistor200. FIG. 3(C) is a cross-sectional view of a portion indicated by adashed-dotted line A3-A4 in FIG. 3(A), and is a cross-sectional view inthe channel width direction of the transistor 200. For clarity of thedrawing, some components are not illustrated in the top view of FIG.3(A).

[Transistor 200]

As illustrated in FIG. 3 , the transistor 200 includes an oxide 230 aprovided over a substrate (not illustrated); an oxide 230 b providedover the oxide 230 a; a conductor 242 a and a conductor 242 b providedover the oxide 230 b to be apart from each other; an insulator 280 whichis provided over the conductor 242 a and the conductor 242 b and inwhich an opening is formed to overlap a region between the conductor 242a and the conductor 242 b; a conductor 260 provided in the opening; aninsulator 250 provided between the conductor 260 and the oxide 230 b,the conductor 242 a, the conductor 242 b, and the insulator 280; and anoxide 230 c provided between the insulator 250 and the oxide 230 b, theconductor 242 a, the conductor 242 b, and the insulator 280. Here, asillustrated in FIGS. 3(B) and 3(C), it is preferable that a top surfaceof the conductor 260 be substantially aligned with the top surfaces ofthe insulator 250, an insulator 254, an insulator 244, the oxide 230 c,and the insulator 280. Hereinafter, the oxide 230 a, the oxide 230 b,and the oxide 230 c may be collectively referred to as an oxide 230. Theconductor 242 a and the conductor 242 b may be collectively referred toas a conductor 242.

Here, the oxide 230 b corresponds to the oxide 12 of the transistor 10 ain the above embodiment. The oxide 230 c corresponds to the oxide 13 ofthe transistor 10 a in the above embodiment. The conductor 242 a and theconductor 242 b correspond to the conductor 14 a and the conductor 14 bof the transistor 10 a in the above embodiment, respectively.

In the transistor 200 illustrated in FIG. 3 , the side surfaces of theconductor 242 a and the conductor 242 b on the conductor 260 side have asubstantially perpendicular shape, as in the transistor 10 a illustratedin FIG. 1(A). Note that the transistor 200 illustrated in FIG. 3 is notlimited thereto, and as in the transistor 10 b illustrated in FIG. 1(B),angles formed by side surfaces and bottom surfaces of the conductor 242a and the conductor 242 b may be greater than or equal to 100 and lessthan or equal to 80°, preferably greater than or equal to 30° and lessthan or equal to 60°. As in the transistor 10 c illustrated in FIG.1(C), the side surfaces of the conductor 242 a and the conductor 242 bthat face each other may have a plurality of surfaces.

As illustrated in FIG. 3 , it is preferable that the insulator 244 andthe insulator 254 be provided between the insulator 280 and theinsulator 224, the oxide 230 a, the oxide 230 b, the conductor 242 a,the conductor 242 b, and the oxide 230 c. Here, as illustrated in FIGS.3(B) and 3(C), the insulator 254 is preferably in contact with the sidesurfaces of the oxide 230 c, the top surface and a side surface of theconductor 242 a, the top surface and a side surface of the conductor 242b, the side surfaces of the oxide 230 a and the oxide 230 b, and a topsurface of the insulator 224. The insulator 244 is preferably providedin contact with a top surface of the insulator 254.

The transistor 200 has, in the region where a channel is formed(hereinafter also referred to as a channel formation region) and itsvicinity, a structure in which three layers of the oxide 230 a, theoxide 230 b, and the oxide 230 c are stacked; however, the presentinvention is not limited thereto. For example, a two-layer structure ofthe oxide 230 b and the oxide 230 c or a stacked-layer structure of fouror more layers may be provided. Each of the oxide 230 a, the oxide 230b, and the oxide 230 c may have a stacked-layer structure of two or morelayers.

For example, when the oxide 230 c has a stacked-layer structureincluding a first oxide and a second oxide over the first oxide, thefirst oxide may have a composition similar to that of the oxide 230 band the second oxide may have a composition similar to that of the oxide230 a.

Although the conductor 260 is shown to have a stacked-layer structure oftwo layers in the transistor 200, the present invention is not limitedthereto. For example, the conductor 260 may have a single-layerstructure or a stacked-layer structure of three or more layers.

Here, the conductor 260 functions as a gate electrode of the transistor,and the conductor 242 a and the conductor 242 b function as a sourceelectrode and a drain electrode. As described above, the conductor 260is formed to be embedded in the opening of the insulator 280 and theregion between the conductor 242 a and the conductor 242 b. Here, thepositions of the conductor 260, the conductor 242 a, and the conductor242 b with respect to the opening of the insulator 280 are selected in aself-aligned manner. That is, in the transistor 200, the gate electrodecan be positioned between the source electrode and the drain electrodein a self-aligned manner. Therefore, the conductor 260 can be formedwithout an alignment margin, resulting in a reduction in the areaoccupied by the transistor 200. Accordingly, miniaturization and highintegration of the semiconductor device can be achieved.

In addition, as illustrated in FIG. 3 , the conductor 260 preferablyincludes a conductor 260 a provided inside the insulator 250 and aconductor 260 b embedded inside the conductor 260 a.

The transistor 200 preferably further includes an insulator 214 providedover the substrate (not illustrated); an insulator 216 provided over theinsulator 214; a conductor 205 provided to be embedded in the insulator216; an insulator 222 provided over the insulator 216 and the conductor205; and the insulator 224 provided over the insulator 222. The oxide230 a is preferably provided over the insulator 224.

An insulator 274 and an insulator 281 functioning as interlayer filmsare preferably provided over the transistor 200. Here, the insulator 274is preferably provided in contact with the top surfaces of the conductor260, the insulator 250, the insulator 254, the insulator 244, the oxide230 c, and the insulator 280.

It is preferable that the insulator 222, the insulator 254, theinsulator 244, and the insulator 274 have a function of inhibitingdiffusion of at least one of hydrogen (e.g., a hydrogen atom, a hydrogenmolecule, and the like). For example, the insulator 222, the insulator254, the insulator 244, and the insulator 274 preferably have a lowerhydrogen permeability than the insulator 224, the insulator 250, and theinsulator 280. Here, it is preferable that the insulator 222, theinsulator 254, the insulator 244, and the insulator 274 have a functionof inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom,an oxygen molecule, and the like). For example, the insulator 222, theinsulator 254, the insulator 244, and the insulator 274 preferably havea lower oxygen permeability than the insulator 224, the insulator 250,and the insulator 280.

Here, the insulator 224, the oxide 230, and the insulator 250 areseparated from the insulator 280 and the insulator 281 by the insulator254, the insulator 244, and the insulator 274. Therefore, impuritiessuch as hydrogen contained in the insulator 280 and the insulator 281and excess oxygen can be inhibited from entering the insulator 224, theoxide 230, and the insulator 250.

Furthermore, it is preferable to provide a conductor 240 (a conductor240 a and a conductor 240 b) functioning as a plug and beingelectrically connected to the transistor 200. Note that an insulator 241(an insulator 241 a and an insulator 241 b) is provided in contact witha side surface of the conductor 240 functioning as a plug. In otherwords, the insulator 241 is provided in contact with an inner wall of anopening in the insulator 254, the insulator 244, the insulator 280, theinsulator 274, and the insulator 281. Alternatively, a first conductorof the conductor 240 may be provided in contact with the side surface ofthe insulator 241 and a second conductor of the conductor 240 may beprovided further inside. Here, a top surface of the conductor 240 and atop surface of the insulator 281 can be substantially level with eachother. Although the transistor 200 having a structure in which the firstconductor of the conductor 240 and the second conductor of the conductor240 are stacked is illustrated, the present invention is not limitedthereto. For example, the conductor 240 may be provided as a singlelayer or to have a stacked-layer structure of three or more layers. Whena component has a stacked-layer structure, the layers may bedistinguished by ordinal numbers corresponding to the formation order.

In the transistor 200, as the oxide 230 (the oxide 230 a, the oxide 230b, and the oxide 230 c), which includes a channel formation region, ametal oxide functioning as an oxide semiconductor (hereinafter alsoreferred to as an oxide semiconductor) is preferably used. Thetransistor 200 using an oxide semiconductor in its channel formationregion has an extremely low leakage current (off-state current) in anon-conduction state; thus, a semiconductor device with low powerconsumption can be provided.

For example, as the oxide 230, a metal oxide such as an In-M-Zn oxide(the element Mis one or more kinds selected from aluminum, gallium,yttrium, tin, copper, vanadium, beryllium, boron, titanium, iron,nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium,hafnium, tantalum, tungsten, magnesium, and the like) is preferablyused. In particular, aluminum, gallium, yttrium, or tin is preferablyused as the element M. Furthermore, as the oxide 230, an In—Ga oxide oran In—Zn oxide may be used.

As described above, the oxide 230 b corresponds to the oxide 12, and theoxide 230 c corresponds to the oxide 13. Accordingly, the oxide 230 bincludes a region where the c-axis is aligned substantiallyperpendicularly to a top surface of the oxide 230 b. The oxide 230 cincludes the region where the c-axis is aligned substantiallyperpendicularly to the top surface of the oxide 230 b, a region wherethe c-axis is aligned substantially perpendicularly to the side surfaceof the conductor 242 a, and a region where the c-axis is alignedsubstantially perpendicularly to the side surface of the conductor 242b. Thus, as in the transistor 10 a described above, the on-statecurrent, the S value, and the frequency characteristic of the transistor200 can be improved. In addition, like the transistor 10 a describedabove, the transistor 200 can have stable electrical characteristics andincreased reliability.

As illustrated in FIG. 3(B), the oxide 230 b may have a smallerthickness in a region that does not overlap with the conductor 242 thanin a region that overlaps with the conductor 242. This is formed whenpart of the top surface of the oxide 230 b is removed at the time offorming the conductor 242 a and the conductor 242 b. When a conductivefilm to be the conductor 242 is deposited, a low-resistance region maybe formed on the top surface of the oxide 230 b in the vicinity of theinterface with the conductive film. Removing the low-resistance regionpositioned between the conductor 242 a and the conductor 242 b on thetop surface of the oxide 230 b in the above manner can prevent formationof the channel in the region.

Accordingly, a semiconductor device including a transistor having a highon-state current can be provided. A semiconductor device including atransistor having high frequency characteristics can be provided. Asemiconductor device which has reduced variation in electricalcharacteristics, stable electrical characteristics, and improvedreliability can be provided. A semiconductor device including atransistor having a low off-state current can be provided.

The structure of the semiconductor device including the transistor 200of one embodiment of the present invention will be described in detailbelow.

The conductor 205 is provided to overlap with the oxide 230 and theconductor 260. Furthermore, the conductor 205 is preferably provided tobe embedded in the insulator 216. Here, the top surface of the conductor205 preferably has favorable planarity. For example, the average surfaceroughness (Ra) of the top surface of the conductor 205 is less than orequal to 1 nm, preferably less than or equal to 0.5 nm, furtherpreferably less than or equal to 0.3 nm. This can achieve favorableplanarity of the insulator 224 formed over the conductor 205 andincrease the crystallinity of the oxide 230 b and the oxide 230 c.

Here, the conductor 260 sometimes functions as a first gate (alsoreferred to as top gate) electrode. The conductor 205 sometimesfunctions as a second gate (also referred to as bottom gate) electrode.In that case, the V_(th) of the transistor 200 can be controlled bychanging a potential applied to the conductor 205 independently of apotential applied to the conductor 260. In particular, the V_(th) of thetransistor 200 can be higher than 0 V and the off-state current can bereduced by applying a negative potential to the conductor 205. Thus, adrain current when a potential applied to the conductor 260 is 0 V canbe lower in the case where a negative potential is applied to theconductor 205 than in the case where the negative potential is notapplied to the conductor 205.

The conductor 205 is preferably larger than the channel formation regionof the oxide 230. As illustrated in FIG. 3(C), it is particularlypreferable that the conductor 205 extend to a region outside an endportion of the oxide 230 that intersects with the channel widthdirection. That is, the conductor 205 and the conductor 260 preferablyoverlap with each other with the insulator therebetween on an outer sideof the side surface of the oxide 230 in the channel width direction.

With the above structure, the channel formation region of the oxide 230can be electrically surrounded by the electric field of the conductor260 functioning as the first gate electrode and the electric field ofthe conductor 205 functioning as the second gate electrode.

As illustrated in FIG. 3(C), the conductor 205 is extended to functionas a wiring. However, without limitation to this, a structure in which aconductor functioning as a wiring is provided under the conductor 205may be employed.

Moreover, a conductive material containing tungsten, copper, or aluminumas its main component is preferably used for the conductor 205. Notethat the conductor 205 is illustrated as a single layer but may have astacked-layer structure, for example, a stacked layer of any of theabove conductive materials and titanium or titanium nitride.

A conductor having a function of inhibiting diffusion of impurities suchas a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogenatom, a nitrogen molecule, a nitrogen oxide molecule (N₂O, NO, NO₂, andthe like), and a copper atom (that does not easily transmit the aboveimpurities) may be used under the conductor 205. Alternatively, it ispreferable to use a conductor which has a function of inhibitingdiffusion of oxygen (e.g., at least one of oxygen atoms, oxygenmolecules, and the like) (that does not easily transmit the aboveoxygen). Note that in this specification, a function of inhibitingdiffusion of impurities or oxygen means a function of inhibitingdiffusion of any one or all of the above impurities and the aboveoxygen.

When a conductor having a function of inhibiting diffusion of oxygen isused under the conductor 205, the conductivity of the conductor 205 canbe inhibited from being reduced because of oxidation of the conductor205. As the conductor having a function of inhibiting diffusion ofoxygen, for example, tantalum, tantalum nitride, ruthenium, or rutheniumoxide is preferably used. Accordingly, a first conductor of theconductor 205 is a single layer or a stacked layer of the aboveconductive materials.

The insulator 214 preferably functions as a barrier insulating film thatprevents impurities such as water or hydrogen from entering thetransistor 200 from the substrate side. Accordingly, for the insulator214, it is preferable to use an insulating material having a function ofinhibiting diffusion of impurities such as a hydrogen atom, a hydrogenmolecule, a water molecule, a nitrogen atom, a nitrogen molecule, anitrogen oxide molecule (e.g., N₂O, NO, and NO₂), and a copper atom(that does not easily transmit the above impurities). Alternatively, itis preferable to use an insulating material having a function ofinhibiting diffusion of oxygen (e.g., at least one of oxygen atoms,oxygen molecules, and the like) (that does not easily transmit the aboveoxygen).

For example, aluminum oxide, silicon nitride, or the like is preferablyused for the insulator 214. Accordingly, impurities such as water andhydrogen can be inhibited from being diffused to the transistor 200 sidefrom the substrate side of the insulator 214. In addition, oxygencontained in the insulator 224 and the like can be prevented from beingdiffused to the substrate side of the insulator 214.

The dielectric constants of the insulator 216, the insulator 280, andthe insulator 281 functioning as an interlayer film is preferably lowerthan that of the insulator 214. When a material with a low dielectricconstant is used for an interlayer film, the parasitic capacitancegenerated between wirings can be reduced. As the insulator 216, theinsulator 280, and the insulator 281, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, silicon oxide to which fluorineis added, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, porous silicon oxide, or the like is usedas appropriate, for example.

The insulator 222 and the insulator 224 each have a function of a gateinsulator.

Here, it is preferable that oxygen be released from the insulator 224 incontact with the oxide 230 by heating. In this specification, oxygenthat is released by heating is referred to as excess oxygen in somecases. For example, silicon oxide, silicon oxynitride, or the like maybe used for the insulator 224 appropriately. When such an insulatorcontaining oxygen is provided in contact with the oxide 230, oxygenvacancies in the oxide 230 can be reduced and the reliability of thetransistor 200 can be improved.

As the insulator 224, specifically, an oxide material from which part ofoxygen is released by heating is preferably used. An oxide from whichoxygen is released by heating is an oxide film in which the amount ofreleased oxygen converted into oxygen atoms is greater than or equal to1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 1.0×10¹⁹atoms/cm³, further preferably greater than or equal to 2.0×10¹⁹atoms/cm³ or greater than or equal to 3.0×10²⁰ atoms/cm³ in TDS (ThermalDesorption Spectroscopy) analysis. Note that the temperature of the filmsurface in the TDS analysis is preferably higher than or equal to 100°C. and lower than or equal to 700° C., or higher than or equal to 100°C. and lower than or equal to 400° C.

The thickness of the region that does not overlap with the oxide 230 bis smaller than the thickness of the other region in some cases. Inparticular, as illustrated in FIG. 3(C), the thickness of the insulator224 in a region that does not overlap with the insulator 254 and theoxide 230 b is significantly smaller than the thickness of the insulator224 in the other region in some cases. The thickness of the insulator224 in the region that does not overlap with the insulator 254 and theoxide 230 b is preferably a thickness with which the above oxygen can besufficiently diffused.

Like the insulator 214 or the like, the insulator 222 preferablyfunctions as a barrier insulating film that inhibits impurities such aswater and hydrogen from entering the transistor 200 from the substrateside. For example, the insulator 222 preferably has a lower hydrogenpermeability than the insulator 224. When the insulator 224, the oxide230, the insulator 250, and the like are surrounded by the insulator222, the insulator 254, the insulator 244, and the insulator 274,impurities such as water and hydrogen can be inhibited from entering thetransistor 200 from outside.

Furthermore, it is preferable that the insulator 222 have a function ofinhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, anoxygen molecule, and the like) (that the insulator 222 do not easilytransmit the above oxygen). For example, the insulator 222 preferablyhas a lower oxygen permeability than the insulator 224. When theinsulator 222 has a function of inhibiting diffusion of oxygen orimpurities, oxygen contained in the oxide 230 can be inhibited frombeing diffused to the substrate side, which is preferable. Furthermore,the conductor 205 can be prevented from reacting with oxygen containedin the insulator 224 and the oxide 230.

As the insulator 222, an insulator containing an oxide of one or both ofaluminum and hafnium, which is an insulating material, is preferablyused. As the insulator containing an oxide of one or both of aluminumand hafnium, aluminum oxide, hafnium oxide, an oxide containing aluminumand hafnium (hafnium aluminate), or the like is preferably used. In thecase where the insulator 222 is formed using such a material, theinsulator 222 functions as a layer that inhibits release of oxygen fromthe oxide 230 and entry of impurities such as hydrogen from theperiphery of the transistor 200 into the oxide 230.

Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobiumoxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, orzirconium oxide may be added to these insulators, for example.Alternatively, these insulators may be subjected to nitriding treatment.Silicon oxide, silicon oxynitride, or silicon nitride may be stackedover the above insulator.

For example, a single layer or a stacked layer of an insulatorcontaining what is called a high-k material such as aluminum oxide,hafnium oxide, tantalum oxide, zirconium oxide, lead zirconate titanate(PZT), strontium titanate (SrTiO₃), or (Ba,Sr)TiO₃ (BST) may be used forthe insulator 222. With miniaturization and high integration oftransistors, a problem such as leakage current may arise because of athinner gate insulator. When a high-k material is used for an insulatorfunctioning as the gate insulator, a gate potential during operation ofthe transistor can be reduced while the physical thickness of the gateinsulator is kept.

Note that the insulator 222 and the insulator 224 may each have astacked-layer structure of two or more layers. In that case, withoutlimitation to a stacked-layer structure formed of the same material, astacked-layer structure formed of different materials may be employed.For example, an insulator similar to the insulator 224 may be providedbelow the insulator 222.

The oxide 230 includes the oxide 230 a, the oxide 230 b over the oxide230 a, and the oxide 230 c over the oxide 230 b. When the oxide 230 a isprovided under the oxide 230 b, impurities can be inhibited from beingdiffused into the oxide 230 b from the structure bodies formed below theoxide 230 a. Moreover, when the oxide 230 c is provided over the oxide230 b, impurities can be inhibited from being diffused into the oxide230 b from the structure bodies formed above the oxide 230 c.

Note that the oxide 230 preferably has a stacked-layer structure ofoxides that differ in the atomic ratio of metal atoms. Specifically, theatomic ratio of the element M to the constituent elements in the metaloxide used for the oxide 230 a is preferably greater than the atomicratio of the element M to the constituent elements in the metal oxideused for the oxide 230 b. Moreover, the atomic ratio of the element M toIn in the metal oxide used for the oxide 230 a is preferably greaterthan the atomic ratio of the element M to In in the metal oxide used forthe oxide 230 b. Furthermore, the atomic ratio of In to the element M inthe metal oxide used for the oxide 230 b is preferably greater than theatomic ratio of In to the element Min the metal oxide used for the oxide230 a. A metal oxide that can be used for the oxide 230 a or the oxide230 b can be used for the oxide 230 c.

As described with reference to FIG. 1 , the oxide 230 b and the oxide230 c preferably have crystallinity, and it is particularly preferableto use a CAAC-OS. An oxide having crystallinity, such as a CAAC-OS, hasa dense structure with small amounts of impurities and defects (oxygenvacancies or the like) and high crystallinity. This can inhibit oxygenextraction from the oxide 230 b by the source electrode or the drainelectrode. This can reduce oxygen extraction from the oxide 230 b evenwhen heat treatment is performed; hence, the transistor 200 is stablewith respect to high temperatures in the manufacturing process (what iscalled thermal budget).

The energy of the conduction band minimum of each of the oxide 230 a andthe oxide 230 c is preferably higher than the energy of the conductionband minimum of the oxide 230 b. In other words, the electron affinityof each of the oxide 230 a and the oxide 230 c is preferably smallerthan the electron affinity of the oxide 230 b. In that case, a metaloxide that can be used for the oxide 230 a is preferably used for theoxide 230 c. Specifically, the atomic ratio of the element M to theconstituent elements in the metal oxide used for the oxide 230 c ispreferably greater than the atomic ratio of the element M to theconstituent elements in the metal oxide used for the oxide 230 b.Moreover, the atomic ratio of the element M to In in the metal oxideused for the oxide 230 c is preferably greater than the atomic ratio ofthe element M to In in the metal oxide used for the oxide 230 b.Furthermore, the atomic ratio of In to the element Min the metal oxideused for the oxide 230 b is preferably greater than the atomic ratio ofIn to the element Min the metal oxide used for the oxide 230 c.

The energy level of the conduction band minimum gradually changes atjunction portions of the oxide 230 a, the oxide 230 b, and the oxide 230c. In other words, the energy level of the conduction band minimum atthe junction portions of the oxide 230 a, the oxide 230 b, and the oxide230 c continuously changes or is continuously connected. To obtain this,the density of defect states in a mixed layer formed at an interfacebetween the oxide 230 a and the oxide 230 b and an interface between theoxide 230 b and the oxide 230 c is preferably made low.

Specifically, when the oxide 230 a and the oxide 230 b or the oxide 230b and the oxide 230 c contain the same element (as a main component) inaddition to oxygen, a mixed layer with a low density of defect statescan be formed. For example, in the case where the oxide 230 b is anIn—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, a gallium oxide, orthe like may be used for the oxide 230 a and the oxide 230 c. The oxide230 c may have a stacked-layer structure. For example, a stacked-layerstructure of an In—Ga—Zn oxide and a Ga—Zn oxide over the In—Ga—Zn oxideor a stacked-layer structure of an In—Ga—Zn oxide and a gallium oxideover the In—Ga—Zn oxide can be used. In other words, a stacked-layerstructure of an In—Ga—Zn oxide and an oxide that does not contain In maybe used for the oxide 230 c.

Specifically, as the oxide 230 a, a metal oxide with In:Ga:Zn=1:3:4[atomic ratio] or In:Ga:Zn=1:1:0.5 [atomic ratio] is used. As the oxide230 b, a metal oxide with In:Ga:Zn=4:2:3 [atomic ratio] orIn:Ga:Zn=3:1:2 [atomic ratio] is used. As the oxide 230 c, a metal oxidewith In:Ga:Zn=1:3:4 [atomic ratio], In:Ga:Zn=4:2:3 [atomic ratio],Ga:Zn=2:1 [atomic ratio], or Ga:Zn=2:5 [atomic ratio] is used. Specificexamples of the oxide 230 c having a stacked-layer structure include astacked-layer structure of In:Ga:Zn=4:2:3 [atomic ratio] andIn:Ga:Zn=1:3:4 [atomic ratio], a stacked-layer structure ofIn:Ga:Zn=4:2:3 [atomic ratio] and Ga:Zn=2:1 [atomic ratio], astacked-layer structure of In:Ga:Zn=4:2:3 [atomic ratio] and Ga:Zn=2:5[atomic ratio], a stacked-layer structure of In:Ga:Zn=4:2:3 [atomicratio] and gallium oxide, and the like.

At this time, the oxide 230 b and the vicinity of the interface serve asa main carrier path. When the oxide 230 a and the oxide 230 c have theabove structure, the density of defect states at the interface betweenthe oxide 230 a and the oxide 230 b and the interface between the oxide230 b and the oxide 230 c can be made low. Thus, the influence ofinterface scattering on carrier conduction is small, and the transistor200 can have a high on-state current and high frequency characteristics.Note that in the case where the oxide 230 c has a stacked-layerstructure, not only an effect of lowering the density of defect state atthe interface between the oxide 230 b and the oxide 230 c but also aneffect of inhibiting diffusion of the constituent elements contained inthe oxide 230 c into the insulator 250 side is expected. Morespecifically, the oxide 230 c has a stacked-layer structure, and theoxide that does not contain In is positioned as an upper layer in thestacked-layer structure, and thus In that can be diffused into theinsulator 250 side can be inhibited. Since the insulator 250 functionsas a gate insulator, the transistor shows poor characteristics when Inis diffused into the insulator 250. Thus, when the oxide 230 c has astacked-layer structure, a semiconductor device having high reliabilitycan be provided.

A metal oxide functioning as an oxide semiconductor is preferably usedfor the oxide 230. For example, as the metal oxide to be the channelformation region of the oxide 230, it is preferable to use one having aband gap of 2 eV or more, preferably 2.5 eV or more. With the use of ametal oxide having such a wide band gap, the off-state current of thetransistor can be reduced. With the use of such a transistor, asemiconductor device with low power consumption can be provided.

The conductor 242 (the conductor 242 a and the conductor 242 b)functioning as the source electrode and the drain electrode is providedover the oxide 230 b. For the conductor 242, it is preferable to use ametal element selected from aluminum, chromium, copper, silver, gold,platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium,vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium,ruthenium, iridium, strontium, and lanthanum; an alloy containing any ofthe above metal elements; an alloy containing a combination of the abovemetal elements; or the like. For example, it is preferable to usetantalum nitride, titanium nitride, tungsten, a nitride containingtitanium and aluminum, a nitride containing tantalum and aluminum,ruthenium oxide, ruthenium nitride, an oxide containing strontium andruthenium, an oxide containing lanthanum and nickel, or the like.Tantalum nitride, titanium nitride, a nitride containing titanium andaluminum, a nitride containing tantalum and aluminum, ruthenium oxide,ruthenium nitride, an oxide containing strontium and ruthenium, and anoxide containing lanthanum and nickel are preferable because they areoxidation-resistant conductive materials or materials that retain theirconductivity even after absorbing oxygen.

When the conductor 242 is provided in contact with the oxide 230, theoxygen concentration of the oxide 230 in the vicinity of the conductor242 sometimes decreases. Furthermore, in the oxide 230 in the vicinityof the conductor 242, a metal compound layer that contains the metalcontained in the conductor 242 and the component of the oxide 230 issometimes formed. In such a case, the carrier density of a region in theoxide 230 in the vicinity of the conductor 242 increases, and the regionbecomes a low-resistance region.

Here, the region between the conductor 242 a and the conductor 242 b isformed to overlap with the opening of the insulator 280. Accordingly,the conductor 260 can be formed between the conductor 242 a and theconductor 242 b in a self-aligned manner.

The insulator 250 functions as a gate insulator. The insulator 250 ispreferably provided in contact with the top surface of the oxide 230 c.For the insulator 250, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, silicon oxide to which fluorine isadded, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, or porous silicon oxide can be used. Inparticular, silicon oxide and silicon oxynitride, which have thermalstability, are preferable.

As in the insulator 224, the concentration of impurities such as waterand hydrogen in the insulator 250 is preferably lowered. The thicknessof the insulator 250 is preferably greater than or equal to 1 nm andless than or equal to 20 nm.

Furthermore, a metal oxide may be provided between the insulator 250 andthe conductor 260. The metal oxide preferably inhibits diffusion ofoxygen from the insulator 250 to the conductor 260. Thus, oxidation ofthe conductor 260 due to oxygen from the insulator 250 can be inhibited.

The metal oxide has a function of part of the gate insulator in somecases. Therefore, when silicon oxide, silicon oxynitride, or the like isused for the insulator 250, a metal oxide that is a high-k material witha high relative permittivity is preferably used as the metal oxide. Whenthe gate insulator has a stacked-layer structure of the insulator 250and the metal oxide, the stacked-layer structure can be thermally stableand have a high relative permittivity. Thus, a gate potential that isapplied during operation of the transistor can be reduced while thephysical thickness of the gate insulator is maintained. In addition, theequivalent oxide thickness (EOT) of the insulator functioning as thegate insulator can be reduced.

Specifically, a metal oxide containing one or more kinds selected fromhafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium,tantalum, nickel, germanium, magnesium, and the like can be used. It isparticularly preferable to use an insulator containing an oxide of oneor both of aluminum and hafnium, for example, aluminum oxide, hafniumoxide, or an oxide containing aluminum and hafnium (hafnium aluminate).

Although the conductor 260 has a two-layer structure in FIG. 3 , theconductor 260 may have a single-layer structure or a stacked-layerstructure of three or more layers.

For the conductor 260 a, it is preferable to use the above conductorhaving a function of inhibiting diffusion of impurities such as ahydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, anitrogen molecule, a nitrogen oxide molecule (N₂O, NO, NO₂, and thelike), and a copper atom. Alternatively, it is preferable to use aconductive material having a function of inhibiting diffusion of oxygen(e.g., at least one of an oxygen atom, an oxygen molecule, and thelike).

When the conductor 260 a has a function of inhibiting diffusion ofoxygen, the conductivity of the conductor 260 b can be inhibited frombeing lowered because of oxidation of the conductor 260 b due to oxygencontained in the insulator 250. As a conductive material having afunction of inhibiting diffusion of oxygen, for example, tantalum,tantalum nitride, ruthenium, ruthenium oxide, or the like is preferablyused.

For the conductor 260 b, it is preferable to use a conductive materialcontaining tungsten, copper, or aluminum as its main component. Theconductor 260 also functions as a wiring, and thus a conductor havinghigh conductivity is preferably used. For example, a conductive materialcontaining tungsten, copper, or aluminum as its main component can beused. The conductor 260 b may have a stacked-layer structure, forexample, a stacked-layer structure of any of the above conductivematerials and titanium or titanium nitride.

As illustrated in FIGS. 3(A) and 3(C), the side surface of the oxide 230is provided to be covered with the conductor 260 in a region of theoxide 230 b that does not overlap with the conductor 242, that is, thechannel formation region of the oxide 230. Accordingly, the electricfield of the conductor 260 functioning as the first gate electrode iseasily applied to the side surface of the oxide 230. Thus, the on-statecurrent of the transistor 200 can be increased, and the frequencycharacteristics can be improved.

Like the insulator 214 or the like, the insulator 254 preferablyfunctions as a barrier insulating film that inhibits impurities such aswater and hydrogen from entering the transistor 200 from the insulator280 side. For example, the insulator 254 preferably has a lower hydrogenpermeability than the insulator 224. Furthermore, as illustrated inFIGS. 3(B) and 3(C), the insulator 254 is preferably in contact with theside surfaces of the oxide 230 c, the top surface and side surface ofthe conductor 242 a, the top surface and side surface of the conductor242 b, the side surfaces of the oxide 230 a, the side surfaces of theoxide 230 b, and the top surface of the insulator 224. Such a structurecan inhibit entry of hydrogen contained in the insulator 280 into theoxide 230 through the top surfaces or side surfaces of the conductor 242a, the conductor 242 b, the oxide 230 a, the oxide 230 b, and theinsulator 224.

Furthermore, it is preferable that the insulator 254 have a function ofinhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, anoxygen molecule, and the like) (that the insulator 254 do not easilytransmit the above oxygen). For example, the insulator 254 preferablyhas a lower oxygen permeability than the insulator 280 or the insulator224.

The insulator 254 is preferably deposited by a sputtering method. Whenthe insulator 254 is deposited by a sputtering method in anoxygen-containing atmosphere, oxygen can be added to a region of theinsulator 224 in contact with the insulator 254 and its vicinity.Accordingly, oxygen can be supplied from the region to the oxide 230through the insulator 224. Here, when the insulator 254 has a functionof inhibiting upward oxygen diffusion, oxygen can be prevented frombeing diffused from the oxide 230 into the insulator 280. Moreover, whenthe insulator 222 has a function of inhibiting downward oxygendiffusion, oxygen can be prevented from being diffused from the oxide230 toward the substrate side. In the above manner, oxygen is suppliedto the channel formation region of the oxide 230. Thus, oxygen vacanciesin the oxide 230 can be reduced, so that the transistor can be inhibitedfrom becoming normally on.

An insulator containing an oxide of one or both of aluminum and hafniumis deposited as the insulator 254, for example. Note that as theinsulator containing an oxide of one or both of aluminum and hafnium,aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium(hafnium aluminate), or the like is preferably used.

Like the insulator 214 or the like, the insulator 244 preferablyfunctions as a barrier insulating film that inhibits impurities such aswater and hydrogen from entering the transistor 200 from the insulator280 side. For example, the insulator 244 preferably has a lower hydrogenpermeability than the insulator 224. Furthermore, as illustrated inFIGS. 3(B) and 3(C), the insulator 244 is preferably provided in contactwith the insulator 254. Such a structure can inhibit entry of hydrogencontained in the insulator 280 into the oxide 230 through the sidesurfaces of the conductor 260, the oxide 230 c, and the insulator 250.

Thus, the insulator 224, the insulator 250, and the oxide 230 arecovered with the insulator 254 and the insulator 244 having a barrierproperty against hydrogen, whereby the insulator 280 is separated fromthe insulator 224, the oxide 230, and the insulator 250 by the insulator254 or the insulator 244. Thus, impurities such as hydrogen can beinhibited from entering the insulator 224, the insulator 250, and theoxide 230 from the outside of the transistor 200, so that the transistor200 can have favorable electrical characteristics and high reliability.

Furthermore, it is preferable that the insulator 244 have a function ofinhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, anoxygen molecule, and the like) (that the insulator 244 do not easilytransmit the above oxygen). For example, the insulator 244 preferablyhas a lower oxygen permeability than the insulator 224. When theinsulator 244 has a function of inhibiting diffusion of oxygen, theconductor 260 can be inhibited from reacting with oxygen contained inthe insulator 280.

An insulator containing aluminum nitride may be used as the insulator244, for example. It is preferable to use, as the insulator 244, anitride insulator that satisfies the composition formula AlN_(x) (x is areal number greater than 0 and less than or equal to 2, preferablygreater than 0.5 and less than or equal to 1.5). In that case, a filmhaving an excellent insulating property and high thermal conductivitycan be obtained, and thus dissipation of heat generated in driving thetransistor 200 can be increased. Aluminum titanium nitride, titaniumnitride, or the like can also be used as the insulator 244. In thatcase, it is preferable to perform deposition by a sputtering methodbecause deposition can be performed without using, as a deposition gas,a gas with high oxidation such as oxygen or ozone. Silicon nitride,silicon nitride oxide, or the like can also be used.

An insulator containing an oxide of one or both of aluminum and hafniumis preferably deposited as the insulator 244, for example. Note that asthe insulator containing an oxide of one or both of aluminum andhafnium, aluminum oxide, hafnium oxide, an oxide containing aluminum andhafnium (hafnium aluminate), or the like is preferably used. In thiscase, the insulator 244 is preferably deposited by an ALD method. An ALDmethod is a deposition method having excellent coverage, and thus canprevent formation of disconnection or the like due to unevenness of theinsulator 244.

The insulator 280 is provided over the insulator 224, the oxide 230, andthe conductor 242 with the insulator 244 and the insulator 254therebetween. For example, as the insulator 280, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon oxide to which fluorine isadded, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, porous silicon oxide, or the like ispreferably included. In particular, silicon oxide and siliconoxynitride, which have thermal stability, are preferable. A materialsuch as silicon oxide, silicon oxynitride, or porous silicon oxide isparticularly preferable because a region including oxygen that isreleased by heating can be easily formed.

Note that the concentration of impurities such as water and hydrogen inthe insulator 280 is preferably lowered. A top surface of the insulator280 may be planarized.

Like the insulator 214 or the like, the insulator 274 preferablyfunctions as a barrier insulating film that inhibits impurities such aswater and hydrogen from entering the insulator 280 from the above. Aninsulator that can be used as the insulator 214, the insulator 254, orthe like is used as the insulator 274, for example.

The insulator 281 functioning as an interlayer film is preferablyprovided over the insulator 274. As in the insulator 224 or the like,the concentration of impurities such as water and hydrogen in the filmof the insulator 281 is preferably lowered.

The conductor 240 a and the conductor 240 b are provided in openingsformed in the insulator 281, the insulator 274, the insulator 280, andthe insulator 244. The conductor 240 a and the conductor 240 b areprovided to face each other with the conductor 260 therebetween. Notethat the level of the top surfaces of the conductor 240 a and theconductor 240 b may be on the same surface as the top surface of theinsulator 281.

Note that the insulator 241 a is provided in contact with the inner wallof the opening of the insulator 281, the insulator 274, the insulator280, the insulator 254, and the insulator 244, and a first conductor ofthe conductor 240 a is formed in contact with a side surface of theinsulator 241 a. The conductor 242 a is located on at least part of thebottom portion of the opening, and thus the conductor 240 a is incontact with the conductor 242 a. Similarly, the insulator 241 b isprovided in contact with the inner wall of the opening of the insulator281, the insulator 274, the insulator 280, the insulator 254, and theinsulator 244, and a first conductor of the conductor 240 b is formed incontact with the side surface of the insulator 241 b. The conductor 242b is located on at least part of the bottom portion of the opening, andthus the conductor 240 b is in contact with the conductor 242 b.

For the conductor 240 a and the conductor 240 b, a conductive materialcontaining tungsten, copper, or aluminum as its main component ispreferably used. The conductor 240 a and the conductor 240 b may have astacked-layer structure.

In the case where the conductor 240 has a stacked-layer structure, theabove conductor having a function of inhibiting diffusion of impuritiessuch as water and hydrogen is preferably used as a conductor in contactwith the oxide 230 a, the oxide 230 b, the conductor 242, the insulator254, the insulator 244, the insulator 280, the insulator 274, and theinsulator 281. For example, tantalum, tantalum nitride, titanium,titanium nitride, ruthenium, ruthenium oxide, or the like is preferablyused. The conductive material having a function of inhibiting diffusionof impurities such as water and hydrogen may be used as a single layeror a stacked layer. The use of the conductive material can preventoxygen added to the insulator 280 from being absorbed by the conductor240 a and the conductor 240 b. Moreover, impurities such as water andhydrogen can be inhibited from entering the oxide 230 through theconductor 240 a and the conductor 240 b from a layer above the insulator281.

As the insulator 241 a and the insulator 241 b, an insulator that can beused as the insulator 244 or the like is used, for example. Since theinsulator 241 a and the insulator 241 b are provided in contact with theinsulator 254 and the insulator 244, impurities such as water andhydrogen can be inhibited from entering the oxide 230 from the insulator280 or the like through the conductor 240 a and the conductor 240 b.Moreover, oxygen contained in the insulator 280 can be prevented frombeing absorbed by the conductor 240 a and the conductor 240 b.

Although not illustrated, a conductor functioning as a wiring may beprovided in contact with the top surface of the conductor 240 a and thetop surface of the conductor 240 b. For the conductor functioning as awiring, a conductive material containing tungsten, copper, or aluminumas its main component is preferably used. The conductor may have astacked-layer structure, for example, a stacked layer of any of theabove conductive materials and titanium or titanium nitride.

The conductor may be formed to be embedded in an opening provided in aninsulator.

<Constituent Material of Semiconductor Device>

Constituent materials that can be used for a semiconductor device willbe described below.

<<Substrate>>

As a substrate over which the transistor 200 is formed, an insulatorsubstrate, a semiconductor substrate, or a conductor substrate may beused, for example. Examples of the insulator substrate include a glasssubstrate, a quartz substrate, a sapphire substrate, a stabilizedzirconia substrate (e.g., an yttria-stabilized zirconia substrate), anda resin substrate. Examples of the semiconductor substrate include asemiconductor substrate of silicon, germanium, or the like and acompound semiconductor substrate containing silicon carbide, silicongermanium, gallium arsenide, indium phosphide, zinc oxide, or galliumoxide. Another example is a semiconductor substrate in which aninsulator region is included in the above semiconductor substrate, e.g.,an SOI (Silicon On Insulator) substrate. Examples of the conductorsubstrate include a graphite substrate, a metal substrate, an alloysubstrate, and a conductive resin substrate. Other examples include asubstrate including a metal nitride and a substrate including a metaloxide. Other examples include an insulator substrate provided with aconductor or a semiconductor, a semiconductor substrate provided with aconductor or an insulator, and a conductor substrate provided with asemiconductor or an insulator. Alternatively, any of these substratesprovided with an element may be used. Examples of the element providedfor the substrate include a capacitor, a resistor, a switching element,a light-emitting element, and a memory element.

<<Insulator>>

An insulating oxide, an insulating nitride, an insulating oxynitride, aninsulating nitride oxide, an insulating metal oxide, an insulating metaloxynitride, and an insulating metal nitride oxide are given as aninsulator.

With miniaturization and high integration of a transistor, for example,a problem of leakage current or the like may arise because a gateinsulator becomes thin. When a high-k material is used for the insulatorfunctioning as a gate insulator, the voltage at the time when thetransistor operates can be reduced while the physical thickness of thegate insulator is kept. By contrast, when a material having a lowrelative permittivity is used for the insulator functioning as aninterlayer film, the parasitic capacitance generated between wirings canbe reduced. Thus, a material is preferably selected depending on thefunction of an insulator.

Examples of the insulator having a high relative permittivity includegallium oxide, hafnium oxide, zirconium oxide, an oxide containingaluminum and hafnium, an oxynitride containing aluminum and hafnium, anoxide containing silicon and hafnium, an oxynitride containing siliconand hafnium, and a nitride containing silicon and hafnium.

Examples of the insulator having a low relative permittivity includesilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, silicon oxide to which fluorine is added, silicon oxide towhich carbon is added, silicon oxide to which carbon and nitrogen areadded, porous silicon oxide, and a resin.

When the transistor using an oxide semiconductor is surrounded by aninsulator that has a function of inhibiting the transmission of oxygenand impurities such as hydrogen (the insulator 214, the insulator 222,the insulator 254, the insulator 244, the insulator 274, and the like),the electrical characteristics of the transistor can be stable. As aninsulator that has a function of inhibiting the transmission of oxygenand impurities such as hydrogen, a single layer or a stacked layer of aninsulator containing, for example, boron, carbon, nitrogen, oxygen,fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon,gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium,or tantalum is used. Specifically, for the insulator having a functionof inhibiting the transmission of oxygen and impurities such ashydrogen, a metal oxide such as aluminum oxide, magnesium oxide, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide; and a metal nitridesuch as aluminum nitride, aluminum titanium nitride, titanium nitride,silicon nitride oxide, or silicon nitride can be used.

Furthermore, the insulator functioning as a gate insulator is preferablyan insulator including a region containing oxygen that is released byheating. When a structure is employed in which silicon oxide or siliconoxynitride including a region containing oxygen that is released byheating is in contact with the oxide 230, oxygen vacancies included inthe oxide 230 can be compensated for.

<<Conductor>>

For the conductor, it is preferable to use a metal element selected fromaluminum, chromium, copper, silver, gold, platinum, tantalum, nickel,titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese,magnesium, zirconium, beryllium, indium, ruthenium, iridium, strontium,lanthanum, and the like; an alloy containing any of the above metalelements; an alloy containing a combination of the above metal elements;or the like. For example, it is preferable to use tantalum nitride,titanium nitride, tungsten, a nitride containing titanium and aluminum,a nitride containing tantalum and aluminum, ruthenium oxide, rutheniumnitride, an oxide containing strontium and ruthenium, an oxidecontaining lanthanum and nickel, or the like. Tantalum nitride, titaniumnitride, a nitride containing titanium and aluminum, a nitridecontaining tantalum and aluminum, ruthenium oxide, ruthenium nitride, anoxide containing strontium and ruthenium, and an oxide containinglanthanum and nickel are preferable because they are oxidation-resistantconductive materials or materials that retain their conductivity evenafter absorbing oxygen. Furthermore, a semiconductor having highelectrical conductivity, typified by polycrystalline silicon containingan impurity element such as phosphorus, or silicide such as nickelsilicide may be used.

Furthermore, a stack including a plurality of conductive layers formedwith the above materials may be used. For example, a stacked-layerstructure combining a material containing the above metal element and aconductive material containing oxygen may be employed. Furthermore, astacked-layer structure combining a material containing the above metalelement and a conductive material containing nitrogen may be employed.Furthermore, a stacked-layer structure combining a material containingthe above metal element, a conductive material containing oxygen, and aconductive material containing nitrogen may be employed.

Note that when an oxide is used in the channel formation region of thetransistor, a stacked-layer structure combining a material containingany of the above-described metal elements and a conductive materialcontaining oxygen is preferably used for the conductor functioning asthe gate electrode. In that case, the conductive material containingoxygen is preferably provided on the channel formation region side. Whenthe conductive material containing oxygen is provided on the channelformation region side, oxygen released from the conductive material iseasily supplied to the channel formation region.

It is particularly preferable to use, for the conductor functioning asthe gate electrode, a conductive material containing oxygen and a metalelement contained in a metal oxide where the channel is formed.Furthermore, a conductive material containing the above metal elementand nitrogen may be used. For example, a conductive material containingnitrogen, such as titanium nitride or tantalum nitride, may be used.Furthermore, indium tin oxide, indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium zincoxide, or indium tin oxide to which silicon is added may be used.Furthermore, indium gallium zinc oxide containing nitrogen may be used.With the use of such a material, hydrogen contained in the metal oxidewhere the channel is formed can be trapped in some cases. Alternatively,hydrogen entering from an external insulator or the like can be trappedin some cases.

<<Metal Oxide>>

As the oxide 230, a metal oxide functioning as an oxide semiconductor ispreferably used. A metal oxide that can be used for the oxide 230 of thepresent invention will be described below.

The metal oxide preferably contains at least indium or zinc. Inparticular, indium and zinc are preferably contained. Furthermore,aluminum, gallium, yttrium, tin, or the like is preferably contained inaddition to them. Furthermore, one or more kinds selected from boron,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likemay be contained.

Here, the case where the metal oxide is an In-M-Zn oxide containingindium, an element M, and zinc is considered. Note that the element M isaluminum, gallium, yttrium, tin, or the like. Other elements that can beused as the element M include boron, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, and magnesium. Note that a plurality of the above-describedelements may be used in combination as the element M.

Note that in this specification and the like, a metal oxide containingnitrogen is also collectively referred to as a metal oxide in somecases. A metal oxide containing nitrogen may be referred to as a metaloxynitride.

[Structure of Metal Oxide]

Oxide semiconductors (metal oxides) can be classified into a singlecrystal oxide semiconductor and anon-single-crystal oxide semiconductor.Examples of the non-single-crystal oxide semiconductors include aCAAC-OS, a polycrystalline oxide semiconductor, an nc-OS(nanocrystalline oxide semiconductor), an amorphous-like oxidesemiconductor (a-like OS), and an amorphous oxide semiconductor.

[Impurities]

Here, the influence of each impurity in the metal oxide will bedescribed.

When the metal oxide contains an alkali metal or an alkaline earthmetal, defect states are formed and carriers are generated in somecases. Thus, a transistor using a metal oxide that contains an alkalimetal or an alkaline earth metal for its channel formation region islikely to have normally-on characteristics. Therefore, it is preferableto reduce the concentration of an alkali metal or an alkaline earthmetal in the metal oxide. Specifically, the concentration of alkalimetal or alkaline earth metal in the metal oxide that is obtained bySIMS (secondary ion mass spectrometry) (the concentration obtained bySIMS) is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower thanor equal to 2×10¹⁶ atoms/cm³.

Furthermore, hydrogen contained in a metal oxide reacts with oxygenbonded to a metal atom to be water, and thus forms an oxygen vacancy insome cases. Entry of hydrogen into the oxygen vacancy generates anelectron serving as a carrier in some cases. Furthermore, in some cases,bonding of part of hydrogen to oxygen bonded to a metal atom causesgeneration of an electron serving as a carrier. Thus, a transistor usingthe metal oxide that contains hydrogen is likely to have normally-oncharacteristics.

Accordingly, hydrogen in the metal oxide is preferably reduced as muchas possible. Specifically, the hydrogen concentration of the metaloxide, which is obtained by SIMS, is set lower than 1×10²⁰ atoms/cm³,preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than5×10¹⁸ atoms/cm³, still further preferably than 1×10¹⁸ atoms/cm³. When ametal oxide in which impurities are sufficiently reduced is used in achannel formation region of a transistor, stable electricalcharacteristics can be given.

Note that as a metal oxide used for a semiconductor of a transistor, athin film having high crystallinity is preferably used. With the use ofthe thin film, the stability or the reliability of the transistor can beimproved. Examples of the thin film include a thin film of asingle-crystal metal oxide and a thin film of a polycrystalline metaloxide. However, to form the thin film of a single-crystal metal oxide orthe thin film of a polycrystalline metal oxide over a substrate, ahigh-temperature process or a laser heating process is needed. Thus, themanufacturing cost is increased, and in addition, the throughput isdecreased.

Non-Patent Document 1 and Non-Patent Document 2 have reported that anIn—Ga—Zn oxide having a CAAC structure (referred to as CAAC-IGZO) wasfound in 2009. It has been reported that CAAC-IGZO has c-axis alignment,a crystal grain boundary is not clearly observed in CAAC-IGZO, andCAAC-IGZO can be formed over a substrate at low temperatures. It hasalso been reported that a transistor using CAAC-IGZO has excellentelectrical characteristics and high reliability.

In addition, in 2013, an In—Ga—Zn oxide having an nc structure (referredto as nc-IGZO) was found (see Non-Patent Document 3). It has beenreported that nc-IGZO has periodic atomic arrangement in a microscopicregion (for example, a region with a size greater than or equal to 1 nmand less than or equal to 3 nm) and there is no regularity of crystalorientation between different regions.

Non-Patent Document 4 and Non-Patent Document 5 have shown a change inaverage crystal size due to electron beam irradiation to thin films ofthe above CAAC-IGZO, the above nc-IGZO, and IGZO having lowcrystallinity. In the thin film of IGZO having low crystallinity,crystalline IGZO with a crystal size of approximately 1 nm was observedeven before the electron beam irradiation. Thus, it has been reportedthat the existence of a completely amorphous structure was not observedin IGZO. In addition, it has been shown that the thin film of CAAC-IGZOand the thin film of nc-IGZO each have higher stability to electron beamirradiation than the thin film of IGZO having low crystallinity. Thus,the thin film of CAAC-IGZO or the thin film of nc-IGZO is preferablyused for a semiconductor of a transistor.

Non-Patent Document 6 shows that a transistor using a metal oxide has anextremely low leakage current in an off state; specifically, theoff-state current per micrometer in the channel width of the transistoris of the order of yA/μm (10²⁴ A/μm). For example, alow-power-consumption CPU utilizing a characteristic of a low leakagecurrent of the transistor using a metal oxide is disclosed (seeNon-Patent Document 7).

Furthermore, application of a transistor using a metal oxide to adisplay device that utilizes the characteristic of a low leakage currentof the transistor has been reported (see Non-Patent Document 8). In thedisplay device, a displayed image is changed several tens of times persecond. The number of times an image is changed per second is referredto as a refresh rate. The refresh rate is also referred to as drivingfrequency. Such high-speed screen change that is hard for human eyes torecognize is considered as a cause of eyestrain. Thus, it is proposedthat the refresh rate of the display device is lowered to reduce thenumber of times of image rewriting. Moreover, driving with a loweredrefresh rate enables the power consumption of the display device to bereduced. Such a driving method is referred to as idling stop (IDS)driving.

The discovery of the CAAC structure and the nc structure has contributedto an improvement in electrical characteristics and reliability of atransistor using a metal oxide having the CAAC structure or the ncstructure, a reduction in manufacturing cost, and an improvement inthroughput. Furthermore, applications of the transistor to a displaydevice and an LSI utilizing the characteristics of low leakage currentof the transistor have been studied.

<Method for Manufacturing Semiconductor Device>

Next, a method for manufacturing a semiconductor device including thetransistor 200 of one embodiment of the present invention, which isillustrated in FIG. 3 , will be described with reference to FIG. 4 toFIG. 9 . In FIG. 4 to FIG. 9 , (A) of each drawing is a top view.Moreover, (B) of each drawing is a cross-sectional view corresponding toa portion indicated by a dashed-dotted line A1-A2 in (A), and is also across-sectional view in the channel length direction of the transistor200. Furthermore, (C) of each drawing is a cross-sectional viewcorresponding to a portion indicated by a dashed-dotted line A3-A4 in(A), and is also a cross-sectional view in the channel width directionof the transistor 200. Note that for simplification of the drawing, somecomponents are not illustrated in the top view in (A) of each drawing.

First, a substrate (not illustrated) is prepared, and the insulator 214is deposited over the substrate. The insulator 214 can be deposited by asputtering method, a chemical vapor deposition (CVD) method, a molecularbeam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, anALD (Atomic Layer Deposition) method, or the like.

Note that CVD methods can be classified into a plasma enhanced CVD(PECVD) method using plasma, a thermal CVD (TCVD) method using heat, aphoto CVD method using light, and the like. Moreover, the CVD methodscan be classified into a metal CVD (MCVD) method and a metal organic CVD(MOCVD) method depending on a source gas to be used.

By a plasma CVD method, a high-quality film can be obtained at arelatively low temperature. Furthermore, a thermal CVD method is adeposition method that does not use plasma and thus enables less plasmadamage to an object. For example, a wiring, an electrode, an element(e.g., a transistor or a capacitor), or the like included in asemiconductor device might be charged up by receiving charges fromplasma. In that case, accumulated charges might break the wiring,electrode, element, or the like included in the semiconductor device. Bycontrast, when a thermal CVD method not using plasma is employed, suchplasma damage is not caused and the yield of the semiconductor devicecan be increased. A thermal CVD method does not cause plasma damageduring deposition, so that a film with few defects can be obtained.

In an ALD method, one atomic layer can be deposited at a time usingself-controllability of atoms; therefore, an ALD method has effects suchas possibilities of deposition of an extremely thin film, deposition ona component with a large aspect ratio, deposition of a film with a smallnumber of defects such as pinholes, deposition with excellent coverage,and low-temperature deposition. An ALD method includes a PEALD (PlasmaEnhanced ALD) method which is a deposition method using plasma. The useof plasma is sometimes preferable because deposition at a lowertemperature is possible. Note that a precursor used in an ALD methodsometimes contains impurities such as carbon. Thus, in some cases, afilm provided by an ALD method contains impurities such as carbon in alarger amount than a film provided by another deposition method. Notethat impurities can be quantified by X-ray photoelectron spectroscopy(XPS).

Unlike in a deposition method in which particles ejected from a targetor the like are deposited, in a CVD method and an ALD method, a film isformed by reaction at a surface of an object. Thus, a CVD method and anALD method are deposition methods that are less likely to be influencedby the shape of an object and thus have favorable step coverage. Inparticular, an ALD method enables excellent step coverage and excellentthickness uniformity and can be favorably used to cover a surface of anopening with a high aspect ratio, for example. On the other hand, an ALDmethod has a relatively low deposition rate; hence, it is sometimespreferable to combine an ALD method with another deposition method witha high deposition rate, such as a CVD method.

A CVD method and an ALD method enable control of the composition of afilm to be obtained with a flow rate ratio of the source gases. Forexample, by a CVD method or an ALD method, a film with a certaincomposition can be deposited depending on the flow rate ratio of thesource gases. Moreover, by a CVD method or an ALD method, by changingthe flow rate ratio of the source gases during the deposition, a filmwhose composition is continuously changed can be deposited. In the casewhere the film is deposited while changing the flow rate ratio of thesource gases, as compared to the case where the film is deposited usinga plurality of deposition chambers, time taken for the deposition can beshortened because time taken for transfer and pressure adjustment isomitted. Thus, the productivity of semiconductor devices can be improvedin some cases.

In this embodiment, for the insulator 214, aluminum oxide is depositedby a sputtering method. The insulator 214 may have a multilayerstructure. For example, a structure may be employed in which aluminumoxide is deposited by a sputtering method and another aluminum oxide isdeposited over the aluminum oxide by an ALD method. Alternatively, astructure may be employed in which aluminum oxide is deposited by an ALDmethod and another aluminum oxide is deposited over the aluminum oxideby a sputtering method.

Then, a conductive film to be the conductor 205 is deposited over theinsulator 214. The conductive film to be the conductor 205 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like. The conductive film to be theconductor 205 can be a multilayer film. In this embodiment, tungsten isdeposited as the conductive film to be the conductor 205.

Next, the conductive film to be the conductor 205 is processed by alithography method, so that the conductor 205 is formed.

In the lithography method, first, a resist is exposed to light through amask. Next, a region exposed to light is removed or left using adeveloping solution, so that a resist mask is formed. Then, etchingthrough the resist mask is conducted, whereby a conductor, asemiconductor, an insulator, or the like can be processed into a desiredshape. The resist mask is formed by, for example, exposure of the resistto light using KrF excimer laser light, ArF excimer laser light, EUV(Extreme Ultraviolet) light, or the like. Alternatively, a liquidimmersion technique may be employed in which a portion between asubstrate and a projection lens is filled with liquid (e.g., water) toperform light exposure. An electron beam or an ion beam may be usedinstead of the above-mentioned light. Note that a mask is not necessaryin the case of using an electron beam or an ion beam. Note that forremoval of the resist mask, dry etching treatment such as ashing can beperformed, wet etching treatment can be performed, wet etching treatmentcan be performed after dry etching treatment, or dry etching treatmentcan be performed after wet etching treatment.

A hard mask formed of an insulator or a conductor may be used instead ofthe resist mask. In the case where a hard mask is used, a hard mask witha desired shape can be formed by forming an insulating film or aconductive film that is the hard mask material over the conductive filmto be the conductor 205, forming a resist mask thereover, and thenetching the hard mask material. The etching of the conductive film to bethe conductor 205 may be performed after removal of the resist mask orwith the resist mask remaining. In the latter case, the resist mask maybe removed during the etching. The hard mask may be removed by etchingafter the etching of the conductive film to be the conductor 205. Thehard mask does not need to be removed in the case where the material ofthe hard mask does not affect the following process or can be utilizedin the following process.

As a dry etching apparatus, a capacitively coupled plasma (CCP) etchingapparatus including parallel plate electrodes can be used. Thecapacitively coupled plasma etching apparatus including the parallelplate electrodes may have a structure in which a high-frequency power isapplied to one of the parallel plate electrodes. Alternatively, astructure may employed in which different high-frequency powers areapplied to one of the parallel plate electrodes. Alternatively, astructure may be employed in which a high-frequency power with the samefrequency is applied to the parallel plate electrodes. Alternatively, astructure may be employed in which high-frequency powers with differentfrequencies are applied to the parallel plate electrodes. Alternatively,a dry etching apparatus including a high-density plasma source can beused. As the dry etching apparatus including a high-density plasmasource, an inductively coupled plasma (ICP) etching apparatus can beused, for example.

Next, an insulating film to be the insulator 216 is deposited over theinsulator 214 and the conductor 205. The insulating film to be theinsulator 216 can be deposited by a sputtering method, a CVD method, anMBE method, a PLD method, an ALD method, or the like. In thisembodiment, as the insulating film to be the insulator 216, siliconoxide is deposited by a CVD method.

Here, the thickness of the insulating film to be the insulator 216 ispreferably greater than or equal to the thickness of the conductor 205.For example, when the thickness of the conductor 205 is 1, the thicknessof the insulating film to be the insulator 216 is greater than or equalto 1 and less than or equal to 3. In this embodiment, the thickness ofthe conductor 205 is 150 nm and the thickness of the insulating film tobe the insulator 216 is 350 nm.

Next, CMP (Chemical Mechanical Polishing) treatment is performed on theinsulating film to be the insulator 216, so that part of the insulatingfilm to be the insulator 216 is removed and a surface of the conductor205 is exposed. In this manner, the insulator 216 and the conductor 205with flat top surfaces can be formed (see FIG. 4 ). The planarity of thetop surfaces of the insulator 216 and the conductor 205 is improved,whereby the crystallinity of a CAAC-OS for forming the oxide 230 b andthe oxide 230 c can be improved.

Note that the method for forming the insulator 216 and the conductor 205is not limited to the above. For example, the conductor 205 may beformed in such a manner that an insulating film to be the insulator 216is deposited over the insulator 214, an opening is formed in theinsulating film, and the conductor 205 is embedded in the opening.

Next, the insulator 222 is deposited over the insulator 216 and theconductor 205. An insulator containing an oxide of one or both ofaluminum and hafnium is preferably deposited for the insulator 222. Notethat as the insulator containing an oxide of one or both of aluminum andhafnium, aluminum oxide, hafnium oxide, an oxide containing aluminum andhafnium (hafnium aluminate), or the like is preferably used. Theinsulator containing an oxide of one or both of aluminum and hafnium hasa barrier property against oxygen, hydrogen, and water. When theinsulator 222 has a barrier property against hydrogen and water,hydrogen and water contained in structure bodies provided around thetransistor 200 are inhibited from being diffused into the transistor 200through the insulator 222, and generation of oxygen vacancies in theoxide 230 can be inhibited.

The insulator 222 can be deposited by a sputtering method, a CVD method,an MBE method, a PLD method, an ALD method, or the like.

Next, an insulating film to be the insulator 224 is deposited over theinsulator 222. The insulating film to be the insulator 224 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like.

Sequentially, heat treatment is preferably performed. The heat treatmentmay be performed at higher than or equal to 250° C. and lower than orequal to 650° C., preferably higher than or equal to 300° C. and lowerthan or equal to 500° C., further preferably higher than or equal to320° C. and lower than or equal to 450° C. Note that the heat treatmentis performed in a nitrogen atmosphere, an inert gas atmosphere, or anatmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or10% or more. The heat treatment may be performed under a reducedpressure. Alternatively, the heat treatment may be performed in such amanner that heat treatment is performed in a nitrogen atmosphere or aninert gas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or10% or more in order to compensate for released oxygen.

In this embodiment, as the heat treatment, treatment is performed in anitrogen atmosphere at 400° C. for one hour after deposition of theinsulator 224. By the heat treatment, impurities such as water andhydrogen contained in the insulator 224 can be removed, for example. Theheat treatment can also be performed after the deposition of theinsulator 222, or the like.

Here, in order to form an excess-oxygen region in the insulator 224,plasma treatment containing oxygen may be performed under a reducedpressure. The plasma treatment containing oxygen is preferably performedusing an apparatus including a power source for generating high-densityplasma using microwaves, for example. Alternatively, a power source forapplying an RF (Radio Frequency) to a substrate side may be included.The use of high-density plasma enables high-density oxygen radicals tobe produced, and RF application to the substrate side allows the oxygenradicals generated by the high-density plasma to be efficientlyintroduced into the insulator 224. Alternatively, after plasma treatmentcontaining an inert gas is performed with this apparatus, plasmatreatment containing oxygen may be performed to compensate for releasedoxygen. Note that impurities such as water and hydrogen contained in theinsulator 224 can be removed by selecting the conditions for the plasmatreatment appropriately. In that case, the heat treatment is notnecessarily performed.

Next, an oxide film 230A to be the oxide 230 a, an oxide film 230B to bethe oxide 230 b, and a conductive film 242A to be the conductor 242 aredeposited in this order over the insulator 224 (see FIG. 4 ). Note thatthe oxide films are preferably deposited successively without beingexposed to an air atmosphere. By the deposition without exposure to theair, impurities or moisture from the air atmosphere can be preventedfrom being attached to the oxide film 230A and the oxide film 230B, sothat the vicinity of an interface between the oxide film 230A and theoxide film 230B can be kept clean.

The oxide film 230A, the oxide film 230B, and the conductive film 242Acan be deposited by a sputtering method, a CVD method, an MBE method, aPLD method, an ALD method, or the like.

In the case where the oxide film 230A and the oxide film 230B aredeposited by a sputtering method, for example, oxygen or a mixed gas ofoxygen and a rare gas is used as a sputtering gas. By increasing theproportion of oxygen contained in the sputtering gas, the amount ofexcess oxygen in the oxide film to be deposited can be increased. In thecase where the above oxide films are deposited by a sputtering method,the above In-M-Zn oxide target can be used.

In particular, when the oxide film 230A is deposited, part of oxygencontained in the sputtering gas is supplied to the insulator 224 in somecases. Therefore, the proportion of oxygen contained in the sputteringgas for the oxide film 230A is preferably 70% or higher, furtherpreferably 80% or higher, still further preferably 100%.

In the case where the oxide film 230B is formed by a sputtering method,when the proportion of oxygen contained in the sputtering gas is 1% orhigher and 30% or lower, preferably 5% or higher and 20% or lower duringthe deposition, an oxygen-deficient oxide semiconductor is formed. In atransistor using an oxygen-deficient oxide semiconductor for its channelformation region, relatively high field-effect mobility can be obtained.In addition, when deposition is performed while the substrate is heated,the crystallinity of the oxide film can be improved. Note that oneembodiment of the present invention is not limited thereto. In the casewhere the oxide film 230B is formed by a sputtering method and theproportion of oxygen contained in the sputtering gas for deposition ishigher than 30% and lower than or equal to 100%, preferably higher thanor equal to 70% and lower than or equal to 100%, an oxygen-excess oxidesemiconductor is formed. In a transistor using an oxygen-excess oxidesemiconductor for its channel formation region, relatively highreliability can be obtained.

In this embodiment, the oxide film 230A is deposited by a sputteringmethod using a target with In:Ga:Zn=1:1:0.5 [atomic ratio] (2:2:1[atomic ratio]) or a target with In:Ga:Zn=1:3:4 [atomic ratio]. Theoxide film 230B is deposited by a sputtering method using a target withIn:Ga:Zn=4:2:4.1 [atomic ratio]. Note that each of the oxide films ispreferably formed in accordance with characteristics required for theoxide 230 by appropriate selection of deposition conditions and anatomic ratio.

In addition, it is preferable that the sputtering gas be highly purifiedfor the deposition of the oxide film 230A and the oxide film 230B. Forexample, as an oxygen gas or a rare gas used as a sputtering gas, a gasthat is highly purified to have a dew point of −60° C. or lower, andpreferably −100° C. or lower is used. When the highly purifiedsputtering gas is used for deposition, entry of moisture or the likeinto the oxide 230 can be prevented as much as possible.

Furthermore, in the case where the oxide film 230A and the oxide film230B are deposited by a sputtering method, moisture in a depositionchamber of a sputtering apparatus is preferably removed as much aspossible. For example, with an adsorption vacuum evacuation pump such asa cryopump, the deposition chamber is preferably evacuated to be a highvacuum state (to a degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa). Inparticular, the partial pressure of gas molecules corresponding to H₂O(gas molecules corresponding to m/z=18) in the deposition chamber in thestandby mode of the sputtering apparatus is preferably lower than orequal to 1×10⁻⁴ Pa, further preferably lower than or equal to 5×10⁻⁵ Pa.

Next, heat treatment may be performed. For the heat treatment, theconditions for the above-described heat treatment can be used. Throughthe heat treatment, impurities such as water and hydrogen in the oxidefilm 230A and the oxide film 230B can be removed, for example. In thisembodiment, treatment is performed at 400° C. in a nitrogen atmospherefor one hour, and successively another treatment is performed at 400° C.in an oxygen atmosphere for one hour.

Next, the oxide film 230A, the oxide film 230B, and the conductive film242A are processed into island shapes to form the oxide 230 a, the oxide230 b, and a conductor layer 242B. Note that in this step, the thicknessof the insulator 224 in a region not overlapping with the oxide 230 a isreduced in some cases (see FIG. 5 ).

Here, the oxide 230 a, the oxide 230 b, and the conductor layer 242B areformed to at least partly overlap with the conductor 205. Alternatively,a structure may be employed in which an angle formed by the oxide 230 a,the oxide 230 b, and the conductor layer 242B and the top surface of theinsulator 222 is a gentle angle. In that case, the angle formed by theside surfaces of the oxide 230 a and the oxide 230 b and the top surfaceof the insulator 222 is preferably greater than or equal to 60° and lessthan 70°. With such a shape, the coverage with the insulator 254 and thelike can be improved in a later step, so that defects such as a void canbe reduced. Alternatively, the side surfaces of the oxide 230 a, theoxide 230 b, and the conductor layer 242B may be substantiallyperpendicular to the top surface of the insulator 222. When the sidesurfaces of the oxide 230 a, the oxide 230 b, and the conductor layer242B are substantially perpendicular to the top surface of the insulator222, the plurality of transistors 200 can be provided in a smaller areaand at a higher density.

The conductor layer 242B has a curved surface between its side surfaceand its top surface. That is, an end portion of the side surface and anend portion of the top surface are preferably curved (such a shape ishereinafter also referred to as a rounded shape). The radius ofcurvature of the curved surface at an end portion of the conductor layer242B layer is greater than or equal to 3 nm and less than or equal to 10nm, preferably greater than or equal to 5 nm and less than or equal to 6nm, for example. When the end portions are not angular, the coveragewith films deposited in a later step can be improved.

Note that the oxide film 230A, the oxide film 230B, and the conductivefilm 242A may be processed by a lithography method. The processing canbe performed by a dry etching method or a wet etching method. Theprocessing by a dry etching method is suitable for microfabrication.

In some cases, the treatment such as dry etching causes the attachmentor diffusion of impurities due to an etching gas or the like to asurface or an inside of the oxide 230 a, the oxide 230 b, or the like.Examples of the impurities include fluorine and chlorine.

In order to remove the above impurities and the like, cleaning isperformed. Examples of the cleaning method include wet cleaning using acleaning solution, plasma treatment using plasma, and cleaning by heattreatment, and any of these cleanings may be performed in appropriatecombination.

The wet cleaning may be performed using an aqueous solution in whichoxalic acid, phosphoric acid, hydrofluoric acid, or the like is dilutedwith carbonated water or pure water. Alternatively, ultrasonic cleaningusing pure water or carbonated water may be performed. In thisembodiment, the ultrasonic cleaning using pure water or carbonated wateris performed.

Sequentially, heat treatment may be performed. For the conditions of theheat treatment, the conditions for the above-described heat treatmentcan be used.

Next, a dummy gate film to be a dummy gate layer 262A is deposited overthe insulator 224, the oxide 230 a, the oxide 230 b, and the conductorlayer 242B.

The dummy gate film to be the dummy gate layer 262A is processed to beused as a dummy gate. The dummy gate is an interim gate electrode. Thatis, the dummy gate film to be the dummy gate layer 262A is processed toform an interim gate electrode, the dummy gate is removed in a laterstep, and a gate electrode is formed using a conductive film or the likeinstead. Thus, a film that is easily microfabricated and easily removedis preferably used as the dummy gate film to be the dummy gate layer262A.

The dummy gate film to be the dummy gate layer 262A can be deposited bya sputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. An insulator, a semiconductor, or a conductor canbe used, for example. Specifically, silicon such as polysilicon,microcrystalline silicon, or amorphous silicon, or a metal film ofaluminum, titanium, tungsten, or the like is used, for example.Alternatively, an SOG (Spin On Glass), a resin film, or the like may beformed by a coating method. For example, a photoresist, polyester,polyolefin, polyamide (nylon, aramid, or the like), polyimide,polycarbonate, acrylic, or the like can be given. When the SOG or theresin film is formed by a coating method, a surface of the dummy gatefilm can be flat. In this manner, when having a flat surface, the dummygate film can be easily microfabricated and easily removed.

The dummy gate film to be the dummy gate layer 262A can be a multilayerfilm using different kinds of films. For example, the dummy gate film tobe the dummy gate layer 262A can have a two-layer structure in which aconductive film and a resin film over the conductive film are formed.When the dummy gate film has such a structure, the conductive filmfunctions as a stopper film for CMP treatment in a later CMP process insome cases, for example. Alternatively, the end of the CMP treatment canbe detected in some cases, so that processing variation can be reducedin some cases.

Next, the dummy gate film to be the dummy gate layer 262A is etched by alithography method to form the dummy gate layer 262A (see FIG. 6 ). Thedummy gate layer 262A is formed to at least partly overlap with theconductor 205 and the oxide 230.

Next, an insulating film 254A is deposited to cover the oxide 230 a, theoxide 230 b, the conductor layer 242B, and the dummy gate layer 262A.Next, an insulating film 244A may be deposited over the insulating film254A (see FIG. 6 ). The insulating film 254A and the insulating film244A can be deposited by a sputtering method, a CVD method, an MBEmethod, a PLD method, an ALD method, or the like.

As the insulating film 254A, an insulating film having a function ofinhibiting diffusion of oxygen and impurities such as hydrogen ispreferably used. For example, an aluminum oxide film is preferablydeposited by a sputtering method. When an aluminum oxide film isdeposited by a sputtering method using a gas containing oxygen, oxygencan be injected into the insulator 224. That is, the insulator 224 cancontain excess oxygen.

As the insulating film 244A, an insulating film having a function ofinhibiting diffusion of oxygen and impurities such as hydrogen ispreferably used. For example, an aluminum oxide film is preferablydeposited by an ALD method. By an ALD method which enables goodcoverage, the insulating film 244A having a uniform thickness can beformed even in a step portion formed by the dummy gate layer 262A andthe like. With the use of an ALD method, a dense thin film can bedeposited. A dense thin film having good coverage can be deposited inthis manner, and thus even when defects such as voids or pinholes aregenerated in the insulating film 254A, for example, they can be coveredwith the insulating film 244A.

As the insulating film 244A, aluminum nitride, silicon nitride, siliconnitride oxide, or the like may be deposited. For example, in the casewhere an aluminum nitride film is deposited by reactive sputtering usingan aluminum target as the insulating film 244A, the ratio of thenitrogen-gas flow rate to the total flow rate of the deposition gas ispreferably higher than or equal to 30% and lower than or equal to 100%,further preferably higher than or equal to 40% and lower than or equalto 100%, still further preferably higher than or equal to 50% and lowerthan or equal to 100%.

As the insulating film 244A, aluminum oxide may be deposited while thesubstrate is being heated at high temperature. The substrate heatingtemperature during the deposition of the insulating film 244A is higherthan or equal to 200° C., preferably higher than or equal to 250° C.,still further preferably higher than or equal to 350° C. Aluminum oxideis deposited as the insulating film 254A by an ALD method at this time,whereby when the insulating film 244A is deposited at the abovetemperature, the dummy gate layer 262A can be prevented from beingdeformed.

Furthermore, fluorine may be added after the deposition of one or bothof the insulating film 244A and the insulating film 254A. The additionof fluorine to one or both of the insulating film 244A and theinsulating film 254A can be performed in such a manner that plasmatreatment is performed in an atmosphere containing a fluorine-based gas(e.g. CF₄) or the like or doping of a gas containing fluorine isperformed. When fluorine is added to one or both of the insulating film244A and the insulating film 254A, hydrogen contained in the film can beexpected to be terminated or gettered by fluorine.

Thus, excess oxygen contained in the insulator 224 can be prevented frombeing diffused to the outside, and in addition, impurities such as waterand hydrogen can be pervented from entering the insulator 224 from theoutside. Note that the deposition of the insulating film 244A can beomitted.

Next, an insulating film to be the insulator 280 is deposited over theinsulating film 244A. The insulating film to be the insulator 280 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like.

Next, the insulating film to be the insulator 280, the dummy gate layer262A, the insulating film 254A, and the insulating film 244A are partlyremoved until part of the dummy gate layer 262A is exposed, and thus,the insulator 280, the dummy gate 262, the insulator 254, and theinsulator 244 are formed (see FIG. 7 ). CMP treatment is preferablyemployed for forming the insulator 280, the dummy gate 262, theinsulator 254, and the insulator 244.

As mentioned above, when the dummy gate layer 262A has a two-layerstructure in which a conductive film and a resin film over theconductive film are formed, for example, in CMP treatment, theconductive film functions as a stopper film for the CMP treatment insome cases. Alternatively, the end of the CMP treatment can be detectedwith the conductive film in some cases, so that variation in the heightof the dummy gate 262 can be reduced in some cases. As illustrated inFIG. 7(B), the top surface of the dummy gate 262 is substantiallyaligned with the top surfaces of the insulator 273, the insulator 274,and the insulator 280.

Next, the dummy gate 262 is removed to form an opening 263 (see FIG. 8). The dummy gate 262 can be removed by wet etching, dry etching,ashing, or the like. Alternatively, a plurality of the above treatmentsmay be performed in combination as appropriate. For example, wet etchingtreatment may be performed after ashing treatment. By removing the dummygate 262, part of a surface of the conductor layer 242B is exposed fromthe opening 263.

Next, by removal of a portion of the conductor layer 242B that isexposed from the opening 263, the surface of the oxide 230 b is partlyexposed, and thus the conductor 242 a and the conductor 242 b can beformed. The removal can be performed by wet etching or dry etching. Inthis embodiment, dry etching is used. Dry etching is preferably usedbecause microfabrication is possible. Here, part of the top surface ofthe oxide 230 b that is exposed between the conductor 242 a and theconductor 242 b is removed in some cases.

At this time, the conductor 242 a and the conductor 242 b are formedusing the insulator 280, the insulator 244, and the insulator 254 asmasks. Thus, the opening 263 formed in the insulator 280, the insulator244, and the insulator 254 overlaps with a region between the conductor242 a and the conductor 242 b. In this manner, the conductor 260 can beformed between the conductor 242 a and the conductor 242 b in aself-aligned manner in a later step.

Next, heat treatment is preferably performed before deposition of anoxide film 230C. The heat treatment may be performed at a temperaturehigher than or equal to 100° C. and lower than or equal to 400° C., forexample, at 200° C. The heat treatment is preferably performed underreduced pressure, for example, may be performed in a vacuum atmosphere.A vacuum atmosphere is kept by evacuation with a turbomolecular pump orthe like. The pressure in the treatment chamber in a vacuum atmosphereis preferably lower than or equal to 1×10⁻² Pa, further preferably lowerthan or equal to 1×10⁻³ Pa.

The oxide film 230C is preferably deposited successively without beingexposed to the air after the heat treatment. For example, it ispreferable that the heat treatment and the deposition be successivelyperformed in different chambers using a multi-chamber depositionapparatus. When such treatment is performed, moisture and hydrogenadsorbed on the surfaces of the surfaces and the like of the oxide 230 aand the oxide 230 b can be removed, and in addition, the concentrationsof moisture and hydrogen in the oxide 230 a and the oxide 230 b can bereduced. Moreover, when the heat treatment and the deposition aresuccessively performed without exposure to the air, impurities such ashydrogen can be prevented from entering the oxide 230 again. Heattreatment similar to the above may also be performed before thedeposition of an insulating film 250A to be described later.

The oxide film 230C can be deposited by a sputtering method, a CVDmethod, an MBE method, a PLD method, an ALD method, or the like. Anoxide film to be the oxide film 230C may be deposited by a depositionmethod similar to that for the oxide film 230A or the oxide film 230B inaccordance with characteristics required for the oxide film 230C. Inthis embodiment, the oxide film 230C is deposited by a sputtering methodusing a target with In:Ga:Zn=1:3:4 [atomic ratio] or In:Ga:Zn=4:2:4.1[atomic ratio].

The oxide film 230C is preferably deposited while the substrate is beingheated. At this time, when the substrate temperature is higher than orequal to 300° C., oxygen vacancies in the oxide film 230B and the oxidefilm 230C can be reduced. For example, the deposition may be performedat the same temperature as the temperature for deposition of theinsulating film 250A to be described later. The deposition is performedwhile the substrate is being heated, whereby the crystallinity of theoxide film 230C and the oxide 230 b can be improved.

In particular, in the deposition of the oxide film 230C, part of oxygencontained in a sputtering gas is supplied to the oxide 230 a and theoxide 230 b, in some cases. Therefore, the proportion of oxygencontained in the sputtering gas for the oxide film 230C is preferably70% or higher, further preferably 80% or higher, still furtherpreferably 100%. In addition, when deposition is performed while thesubstrate is heated, the crystallinity of the oxide film can beimproved.

Next, the insulating film 250A is deposited. The insulating film 250Acan be deposited by a sputtering method, a CVD method, an MBE method, aPLD method, an ALD method, or the like. For the insulating film 250A,silicon oxynitride is preferably deposited by a CVD method. Note thatthe deposition temperature at the time of the deposition of theinsulating film 250A is preferably higher than or equal to 300° C. andlower than 450° C., further preferably higher than or equal to 300° C.and lower than 400° C., particularly preferably approximately 350° C.When the insulating film 250A is deposited at 350° C., for example, aninsulator having few impurities can be deposited.

Note that oxygen is excited by microwaves to generate high-densityoxygen plasma, and the insulating film 250A is exposed to the oxygenplasma, whereby oxygen can be introduced into the insulating film 250A.

Furthermore, heat treatment may be performed. For the heat treatment,the conditions for the above-described heat treatment can be used. Bythe heat treatment, the concentrations of moisture and hydrogen in theinsulating film 250A can be reduced.

Next, a conductive film 260Aa and a conductive film 260Ab are deposited.The conductive film 260Aa and the conductive film 260Ab can be depositedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like. A CVD method is preferably used, for example.In this embodiment, the conductive film 260Aa is deposited by an ALDmethod, and the conductive film 260Ab is deposited by a CVD method (seeFIG. 8 ).

Then, the oxide film 230C, the insulating film 250A, the conductive film260Aa, and the conductive film 260Ab are polished by CMP treatment untilthe insulator 280 is exposed, whereby the oxide 230 c, the insulator250, and the conductor 260 (the conductor 260 a and the conductor 260 b)are formed (see FIG. 9 ).

Next, heat treatment may be performed. For the heat treatment, theconditions for the above-described heat treatment can be used. By theheat treatment, the concentrations of moisture and hydrogen in theinsulator 280 can be reduced.

Next, an insulating film to be the insulator 274 is formed over theinsulator 280 (see FIG. 9 ). The insulating film to be the insulator 274can be deposited by a sputtering method, a CVD method, an MBE method, aPLD method, an ALD method, or the like. An aluminum oxide film ispreferably deposited as the insulating film to be the insulator 274 by asputtering method, for example. When an aluminum oxide film is depositedby a sputtering method, hydrogen contained in the insulator 280 can beprevented from being diffused into the oxide 230 in some cases (see FIG.9 ).

Next, heat treatment may be performed. For the heat treatment, theconditions for the above-described heat treatment can be used. By theheat treatment, the concentrations of moisture and hydrogen in theinsulator 280 can be reduced.

Next, an insulating film to be the insulator 281 may be deposited overthe insulator 274. The insulating film to be the insulator 281 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like (see FIG. 9 ).

Next, openings reaching the conductor 242 a and the conductor 242 b areformed in the insulator 254, the insulator 244, the insulator 280, theinsulator 274, and the insulator 281. The openings may be formed by alithography method.

Next, an insulating film to be the insulator 241 is deposited, and theinsulating film is subjected to anisotropic etching to form theinsulator 241. The insulating film can be deposited by a sputteringmethod, a CVD method, an MBE method, a PLD method, an ALD method, or thelike. As the insulating film to be the insulator 241, an insulating filmhaving a function of inhibiting the passage of oxygen is preferablyused. For example, an aluminum oxide film is preferably deposited by anALD method. For the anisotropic etching, a dry etching method may beemployed, for example. By forming aluminum oxide on the side wallportions of the openings, the transmission of oxygen from the outsidecan be inhibited and oxidation of the conductor 240 a and the conductor240 b can be prevented. Furthermore, impurities such as water andhydrogen can be prevented from being diffused from the conductor 240 aand the conductor 240 b to the outside.

Next, a conductive film to be the conductor 240 a and the conductor 240b is deposited. The conductive film to be the conductor 240 a and theconductor 240 b desirably has a stacked-layer structure which includes aconductor having a function of inhibiting diffusion of impurities suchas water and hydrogen. For example, a stacked layer of tantalum nitride,titanium nitride, or the like and tungsten, molybdenum, copper, or thelike can be employed. The conductive film to be the conductor 240 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like.

Next, CMP treatment is performed to remove part of the conductive filmto be the conductor 240 a and the conductor 240 b, so that the insulator281 is exposed. As a result, the conductive film remains only in theopenings, so that the conductor 240 a and the conductor 240 b havingflat top surfaces can be formed (see FIG. 3 ). Note that the insulator281 is partly removed by the CMP treatment in some cases.

Through the above process, the semiconductor device including thetransistor 200 illustrated in FIG. 3 can be manufactured. As illustratedin FIG. 4 to FIG. 9 , with the use of the method for manufacturing thesemiconductor device described in this embodiment, the transistor 200can be formed.

According to one embodiment of the present invention, a semiconductordevice having a high on-state current can be provided. According to oneembodiment of the present invention, a semiconductor device having highfrequency characteristics can be provided. According to one embodimentof the present invention, a semiconductor device with favorablereliability can be provided. According to one embodiment of the presentinvention, a semiconductor device that can be miniaturized or highlyintegrated can be provided. Alternatively, according to one embodimentof the present invention, a semiconductor device having favorableelectrical characteristics can be provided. Alternatively, according toone embodiment of the present invention, a semiconductor device having alow off-state current can be provided. Alternatively, according to oneembodiment of the present invention, a semiconductor device with lowpower consumption can be provided. According to one embodiment of thepresent invention, a semiconductor device with high productivity can beprovided.

The structure, method, and the like described above in this embodimentcan be used in combination as appropriate with the structures, methods,and the like described in the other embodiments.

Modification Example of Semiconductor Device

An example of a semiconductor device including the transistor 200 of oneembodiment of the present invention which is different from thesemiconductor device described in <Structure example of semiconductordevice> above and an example of a method for manufacturing thesemiconductor device will be described below with reference to FIG. 10to FIG. 20 .

In FIG. 10 to FIG. 20 , (A) of each drawing is a top view. Moreover, (B)of each drawing is a cross-sectional view corresponding to a portionindicated by a dashed-dotted line A1-A2 in FIG. 10(A), and is also across-sectional view in the channel length direction of the transistor200. Furthermore, (C) of each drawing is a cross-sectional viewcorresponding to a portion indicated by a dashed-dotted line A3-A4 inFIG. 10(A), and is also a cross-sectional view in the channel widthdirection of the transistor 200. For clarity of the drawing, somecomponents are not illustrated in the top view of (A) of each drawing.

Note that in the semiconductor device illustrated in FIG. 10 to FIG. 20, components having the same functions as the components in thesemiconductor device described in <Structure example of semiconductordevice> (see FIG. 3 ) are denoted by the same reference numerals. Notethat in this section, the materials described in detail in <Structureexample of semiconductor device> can be used as the constituentmaterials for the transistor 200.

The transistor 200 illustrated in FIG. 10 is different from thetransistor 200 illustrated in FIG. 3 in that the insulator 244 is notincluded and the insulator 254 is provided between the insulator 280 andthe insulator 224, the oxide 230 a, the oxide 230 b, the conductor 242a, and the conductor 242 b. That is, the transistor 200 illustrated inFIG. 10 has a structure in which the side surface of the oxide 230 c isin contact with the insulator 280.

In addition, in the transistor 200 illustrated in FIG. 10 , the sidesurfaces of the conductor 242 a and the conductor 242 b on the conductor260 side have a tapered shape, as in the transistor 10 b illustrated inFIG. 1(B). Here, the angles formed by the side surfaces and the bottomsurfaces of the conductor 242 a and the conductor 242 b may be greaterthan or equal to 100 and less than or equal to 80°, preferably greaterthan or equal to 300 and less than or equal to 60°. Thus, contributionof the electric field of the conductor 260 can be increased also in aregion of the oxide 230 in the vicinity of the conductor 242 a and theconductor 242 b, and the on-state current and the frequencycharacteristics of the transistor 200 can be improved. Note that thetransistor 200 illustrated in FIG. 10 is not limited to this, and as inthe transistor 10 a illustrated in FIG. 1(A), the side surfaces of theconductor 242 a and the conductor 242 b may be substantiallyperpendicular to the bottom surfaces. As in the transistor 10 cillustrated in FIG. 1(C), the side surfaces of the conductor 242 a andthe conductor 242 b that face each other may have a plurality ofsurfaces.

The semiconductor device illustrated in FIG. 10 is preferable because itcan be manufactured without forming the dummy gate 262 and thus themanufacturing process of the semiconductor device can be simplified.

The method for manufacturing the semiconductor device illustrated inFIG. 10 up to the formation of the oxide 230 a, the oxide 230 b, and theconductor layer 242B is similar to the method for manufacturing thesemiconductor device illustrated in FIG. 3 . Therefore, the method formanufacturing the semiconductor device in FIG. 4 and FIG. 5 can bereferred to.

Then, the insulating film 254A is deposited over the insulator 224, theoxide 230 a, the oxide 230 b, and the conductor layer 242B.

Next, an insulating film to be the insulator 280 is deposited over theinsulating film 254A. The insulating film to be the insulator 280 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like. Next, the insulating film to be theinsulator 280 is subjected to CMP treatment, so that the insulator 280having a flat top surface is formed.

Next, part of the insulator 280, part of the insulating film 254A, andpart of the conductor layer 242B are removed to form an opening 264reaching the oxide 230 b (see FIG. 11 ). The opening is preferablyformed to overlap with the oxide 230 a, the oxide 230 b, and theconductor 205. The conductor 242 a, the conductor 242 b, and theinsulator 254 are formed by forming the opening 264. Here, the part ofthe top surface of the oxide 230 b that is exposed between the conductor242 a and the conductor 242 b is removed in some cases.

The opening 264 may be formed by wet etching; however, dry etching ispreferably used because microfabrication is possible. The opening 264 ispreferably formed with a hard mask formed over the insulator 280. As thehard mask, a conductor may be used or an insulator may be used.

The part of the insulator 280, the part of the insulating film 254A, andthe part of the conductor may be processed under different conditions.For example, the part of the insulator 280 may be processed by a dryetching method, the part of the insulating film 254A may be processed bya wet etching method, and the part of the conductor layer 242B may beprocessed by a dry etching method.

The following steps in the method for manufacturing the semiconductordevice illustrated in FIG. 10 are similar to those in the method formanufacturing the semiconductor device illustrated in FIG. 3 .Therefore, the method for manufacturing the semiconductor device in FIG.8 and FIG. 9 can be referred to.

The transistor 200 illustrated in FIG. 12 is different from thetransistor 200 illustrated in FIG. 3 in that a conductor 243 a isprovided between the conductor 242 a and the oxide 230 b and a conductor243 b is provided between the conductor 242 b and the oxide 230 b. Here,the conductor 242 a (the conductor 242 b) is provided in contact with atop surface and a side surface on the conductor 260 side of theconductor 243 a (the conductor 243 b) and the top surface of the oxide230 b. Here, as the conductor 243, a conductor that can be used as theconductor 242 may be used. Furthermore, the thickness of the conductor243 is preferably larger than at least that of the conductor 242.

The transistor 200 illustrated in FIG. 12 is different from thetransistor 200 illustrated in FIG. 3 in that the distance between theconductor 242 a and the conductor 242 b is shorter than the length ofthe opening 263 formed in the insulator 280, the insulator 244, and theinsulator 254 in the channel length direction.

When the transistor 200 illustrated in FIG. 12 has the above structure,the contribution of the electric field of the conductor 260 can beincreased also in the region of the oxide 230 in the vicinity of theconductor 242 a and the conductor 242 b. Thus, the substantial channellength of the transistor 200 can be shortened, so that the on-statecurrent and the frequency characteristics can be improved.

The conductor 243 a (the conductor 243 b) is preferably provided tooverlap with the conductor 240 a (the conductor 240 b). With thisstructure, the conductor 243 a (the conductor 243 b) is provided in abottom portion of the opening, and thus the oxide 230 b can be preventedfrom being overetched in the etching for forming the opening where theconductor 240 a (the conductor 240 b) is embedded.

In the transistor 200 illustrated in FIG. 12 , the side surfaces of theconductor 242 a and the conductor 242 b on the conductor 260 side have asubstantially perpendicular shape, as in the transistor 10 a illustratedin FIG. 1(A). Note that the transistor 200 illustrated in FIG. 12 is notlimited thereto, and as in the transistor 10 b illustrated in FIG. 1(B),angles formed by side surfaces and bottom surfaces of the conductor 242a and the conductor 242 b may be greater than or equal to 100 and lessthan or equal to 80°, preferably greater than or equal to 30° and lessthan or equal to 60°. As in the transistor 10 c illustrated in FIG.1(C), the side surfaces of the conductor 242 a and the conductor 242 bthat face each other may have a plurality of surfaces.

The method for manufacturing the semiconductor device illustrated inFIG. 12 up to the deposition of the oxide film 230B to be the oxide 230b is similar to the method for manufacturing the semiconductor deviceillustrated in FIG. 3 . Therefore, the method for manufacturing thesemiconductor device in FIG. 4 can be referred to.

Next, a conductive film to be the conductor layer 243A is deposited overthe oxide film 230B. The conductive film to be the conductor layer 243Acan be deposited by a sputtering method, a CVD method, an MBE method, aPLD method, an ALD method, or the like.

Next, part of the conductive film to be the conductor layer 243A isremoved to form the conductor layer 243A (see FIG. 13 ). Although theconductor layer 243A has an opening in FIG. 13 , this embodiment is notlimited to this. In this step, a portion of the conductive film to bethe conductor layer 243A that corresponds to a region between theconductor 243 a and the conductor 243 b is removed. For example, theconductive film to be the conductor layer 243A may be divided into anisland-shaped conductor corresponding to the conductor 243 a and anisland-shaped conductor corresponding to the conductor 243 b.

The conductive film to be the conductor layer 243A may be processed by alithography method. The processing can be performed by a dry etchingmethod or a wet etching method. The processing by a dry etching methodis suitable for microfabrication.

Next, the conductive film 242A is deposited over the oxide 230 b and theconductor layer 243A. For the deposition of the conductive film 242A,the method for manufacturing the semiconductor device in FIG. 5 can bereferred to.

The following steps up to the formation of the dummy gate layer 262A andthe deposition of the insulating film 254A and the insulating film 244A(see FIG. 14 ) are similar to those in the method for manufacturing thesemiconductor device illustrated in FIG. 3 . Therefore, the method formanufacturing the semiconductor device in FIG. 5 and FIG. 6 can bereferred to.

For example, aluminum oxide may be deposited as the insulating film 244Awhile the substrate is being heated at high temperature. The substrateheating temperature in the deposition of the insulating film 244A ishigher than or equal to 200° C., preferably higher than or equal to 250°C., further preferably higher than or equal to 350° C. By deposition ofaluminum oxide as the insulating film 254A by an ALD method at thistime, when the insulating film 244A is deposited at the abovetemperature, the dummy gate layer 262A can be prevented from beingdeformed.

Furthermore, fluorine may be added after the deposition of one or bothof the insulating film 244A and the insulating film 254A. The additionof fluorine to one or both of the insulating film 244A and theinsulating film 254A can be performed in such a manner that plasmatreatment is performed in an atmosphere containing a fluorine-based gas(e.g. CF₄) or a gas containing fluorine is doped. When fluorine is addedto one or both of the insulating film 244A and the insulating film 254A,hydrogen contained in the film can be expected to be terminated orgettered by fluorine.

Next, the insulating film to be the insulator 280 is deposited over theinsulator 244A. The insulating film to be the insulator 280 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like.

Next, the insulating film to be the insulator 280, the dummy gate layer262A, the insulating film 254A, and the insulating film 244A are partlyremoved until part of the dummy gate layer 262A is exposed, and thus,the insulator 280, the dummy gate 262, the insulator 254, and theinsulator 244 are formed. CMP treatment is preferably employed forforming the insulator 280, the dummy gate 262, the insulator 254, andthe insulator 244. The method for manufacturing a semiconductor devicein FIG. 7 can be referred to for the details.

Next, the dummy gate 262 is removed to form the opening 263 (see FIG. 8). The dummy gate 262 can be removed by wet etching, dry etching,ashing, or the like. Alternatively, a plurality of the above treatmentsmay be performed in combination as appropriate. For example, wet etchingtreatment may be performed after ashing treatment. By removing the dummygate 262, part of the surface of the conductor layer 242B is exposedfrom the opening 263.

Next, a dummy film 265 is deposited over the insulator 280, theinsulator 244, the insulator 254, and the conductor layer 242B (see FIG.15 ). The dummy film 265 needs to be deposited on a side wall of theopening 263, and thus the distance between the conductor 242 a and theconductor 242 b, that is, the substantial channel length depends on thethickness of the dummy film. Therefore, the dummy film 265 is preferablydeposited by an ALD method with which high coverage can be achieved andminor adjustment on the thickness is relatively easy. The thickness ofthe dummy film 265 may be set as appropriate in accordance with requiredelectrical characteristics of the transistor 200; when the thickness isset to 5 nm, for example, the channel length can be substantiallyreduced by 10 nm. Note that the dummy film 265 is finally removed;therefore, it is preferable to use a film that is easily microfabricatedand easily removed.

Next, the dummy film 265 is subjected to anisotropic etching, and only aportion of the dummy film 265 in contact with the side wall of theopening 263 remains. Moreover, the conductor layer 242B is etched usingthe remaining dummy film 265 as a mask to form the conductor 242 a andthe conductor 242 b (see FIG. 16 ). Note that the etching of the dummyfilm 265 and the etching of the conductor layer 242B may be performedsuccessively. The part of the top surface of the oxide 230 b that isexposed between the conductor 242 a and the conductor 242 b is removedin some cases.

At this time, the conductor 242 a and the conductor 242 b are formedusing the remaining dummy film 265 as a mask. Thus, the opening 263formed in the insulator 280, the insulator 244, and the insulator 254overlaps with the region between the conductor 242 a and the conductor242 b. In this manner, the conductor 260 can be formed between theconductor 242 a and the conductor 242 b in a self-aligned manner in alater step.

Next, the remaining dummy film 265 is selectively removed by isotropicetching (see FIG. 17 ). As the isotropic etching, wet etching or etchingusing a reactive gas is performed, for example. In this manner, thedistance between the conductor 242 a and the conductor 242 b can be madeshorter than the length of the opening 263 in the channel lengthdirection.

The following steps in the method for manufacturing the semiconductordevice illustrated in FIG. 12 are similar to those in the method formanufacturing the semiconductor device illustrated in FIG. 3 .Therefore, the method for manufacturing the semiconductor device in FIG.8 and FIG. 9 can be referred to.

The transistor 200 illustrated in FIG. 18 is different from thetransistor 200 illustrated in FIG. 12 in that the insulator 244 is notincluded and the insulator 254 is provided between the insulator 280 andthe insulator 224, the oxide 230 a, the oxide 230 b, the conductor 243a, the conductor 243 b, the conductor 242 a, and the conductor 242 b.That is, the transistor 200 illustrated in FIG. 18 has a structure inwhich the side surface of the oxide 230 c is in contact with theinsulator 280.

In the transistor 200 illustrated in FIG. 18 , the side surfaces of theconductor 242 a and the conductor 242 b on the conductor 260 side have asubstantially perpendicular shape, as in the transistor 10 a illustratedin FIG. 1(A). Note that the transistor 200 illustrated in FIG. 18 is notlimited thereto, and as in the transistor 10 b illustrated in FIG. 1(B),angles formed by side surfaces and bottom surfaces of the conductor 242a and the conductor 242 b may be greater than or equal to 10° and lessthan or equal to 80°, preferably greater than or equal to 30° and lessthan or equal to 60°. As in the transistor 10 c illustrated in FIG.1(C), the side surfaces of the conductor 242 a and the conductor 242 bthat face each other may have a plurality of surfaces.

The semiconductor device illustrated in FIG. 18 is preferable because itcan be manufactured without forming the dummy gate 262 and thus themanufacturing process of the semiconductor device can be simplified.

The method for manufacturing the semiconductor device illustrated inFIG. 18 up to the formation of the oxide 230 a, the oxide 230 b, theconductor 242 a, the conductor 242 b, and the conductor layer 242B issimilar to the method for manufacturing the semiconductor deviceillustrated in FIG. 12 . Therefore, the methods for manufacturing thesemiconductor devices in FIG. 4 , FIG. 5 , and FIG. 13 can be referredto.

Then, the insulating film 254A is deposited over the insulator 224, theoxide 230 a, the oxide 230 b, the conductor 243 a, the conductor 243 b,and the conductor layer 242B.

Next, the insulating film to be the insulator 280 is deposited over theinsulating film 254A. The insulating film to be the insulator 280 can bedeposited by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like. Next, the insulating film to be theinsulator 280 is subjected to CMP treatment, so that the insulator 280having a flat top surface is formed.

Next, the part of the insulator 280, the part of the insulating film254A, and the part of the conductor layer 242B are removed to form anopening 264 a reaching the oxide 230 b (see FIG. 19 ). The opening ispreferably formed to overlap with the oxide 230 a, the oxide 230 b, andthe conductor 205. The conductor 242 a, the conductor 242 b, and theinsulator 254 are formed by forming the opening 264 a. Here, the part ofthe top surface of the oxide 230 b that is exposed between the conductor242 a and the conductor 242 b is removed in some cases.

The opening 264 a may be formed by wet etching; however, dry etching ispreferably used because microfabrication is possible. The opening 264 ais preferably formed with a hard mask formed over the insulator 280. Asthe hard mask, a conductor may be used or an insulator may be used.

The part of the insulator 280, the part of the insulating film 254A, andthe part of the conductor may be processed under different conditions.For example, the part of the insulator 280 may be processed by a dryetching method, the part of the insulating film 254A may be processed bya wet etching method, and the part of the conductor layer 242B may beprocessed by a dry etching method.

Next, the opening 264 a in the insulator 280 is subjected to sideetching to form an opening 264 b (see FIG. 20 ). As the side etching ofthe insulator 280, isotropic etching such as wet etching or etchingusing a reactive gas may be used. For example, a side wall of theopening 264 b is recessed from the opening 264 a by 5 nm, whereby theconductor 242 a (the conductor 242 b) can be protruded from the sidewall of the opening 264 b by 5 nm. In this manner, the distance betweenthe conductor 242 a and the conductor 242 b can be shorter than thelength of the opening 264 b in the channel length direction.

The following steps in the method for manufacturing the semiconductordevice illustrated in FIG. 18 are similar to those in the method formanufacturing the semiconductor device illustrated in FIG. 3 .Therefore, the method for manufacturing the semiconductor device in FIG.8 and FIG. 9 can be referred to.

The structure, composition, method, and the like described above in thisembodiment can be used in appropriate combination with the structures,compositions, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device will bedescribed with reference to FIG. 21 and FIG. 22 .

[Memory Device 1]

FIG. 21 illustrates an example of a semiconductor device (memory device)in which the capacitor of one embodiment of the present invention isused. In the semiconductor device of one embodiment of the presentinvention, the transistor 200 is provided above a transistor 300, and acapacitor 100 is provided above the transistor 300 and the transistor200. Note that the transistor 200 described in the above embodiment canbe used as the transistor 200, for example.

The transistor 200 is a transistor whose channel is formed in asemiconductor layer containing an oxide semiconductor. Since thetransistor 200 has a low off-state current, a memory device includingthe transistor 200 can retain stored data for a long time. In otherwords, such a memory device does not require refresh operation or has anextremely low frequency of the refresh operation, which leads to asufficient reduction in power consumption of the memory device.

In the semiconductor device illustrated in FIG. 21 , a wiring 1001 iselectrically connected to a source of the transistor 300, and a wiring1002 is electrically connected to a drain of the transistor 300. Awiring 1003 is electrically connected to one of the source and the drainof the transistor 200. A wiring 1004 is electrically connected to afirst gate of the transistor 200. A wiring 1006 is electricallyconnected to a second gate of the transistor 200. Agate of thetransistor 300 and the other of the source and the drain of thetransistor 200 are electrically connected to one electrode of thecapacitor 100. A wiring 1005 is electrically connected to the otherelectrode of the capacitor 100.

By arranging the memory devices illustrated in FIG. 21 in a matrix, amemory cell array can be formed.

<Transistor 300>

The transistor 300 is provided over a substrate 311 and includes aconductor 316 functioning as a gate electrode, an insulator 315functioning as a gate insulator, a semiconductor region 313 that is apart of the substrate 311, and a low-resistance region 314 a and alow-resistance region 314 b functioning as the source region and thedrain region. The transistor 300 is of either a p-channel type or ann-channel type.

In the transistor 300 illustrated in FIG. 21 , the semiconductor region313 (part of the substrate 311) in which a channel is formed has aconvex shape. Furthermore, the conductor 316 is provided so as to covera side surface and top surface of the semiconductor region 313 with theinsulator 315 positioned therebetween. Note that a material foradjusting the work function may be used for the conductor 316. Such atransistor 300 is also referred to as a FIN-type transistor because itutilizes a convex portion of the semiconductor substrate. Note that aninsulator functioning as a mask for forming the convex portion may beplaced in contact with an upper portion of the convex portion.Furthermore, although the case where the convex portion is formed byprocessing part of the semiconductor substrate is described here, asemiconductor film having a convex shape may be formed by processing anSOI substrate.

Note that the transistor 300 illustrated in FIG. 21 is an example andthe structure is not limited thereto; an appropriate transistor is usedin accordance with a circuit configuration or a driving method.

<Capacitor 100>

The capacitor 100 is provided above the transistor 200. The capacitor100 includes a conductor 110 functioning as a first electrode, aconductor 120 functioning as a second electrode, and an insulator 130functioning as a dielectric.

For example, a conductor 112 and the conductor 110 over the conductor246 can be formed at the same time. Note that the conductor 112 has afunction of a plug or a wiring that is electrically connected to thecapacitor 100, the transistor 200, or the transistor 300.

Although the conductor 112 and the conductor 110 having a single-layerstructure are illustrated in FIG. 21 , the structure is not limitedthereto; a stacked-layer structure of two or more layers may beemployed. For example, between a conductor having a barrier property anda conductor having high conductivity, a conductor which is highlyadhesive to the conductor having a barrier property and the conductorhaving high conductivity may be formed.

The insulator 130 can be provided to have a single-layer structure or astacked-layer structure using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafniumoxide, hafnium oxynitride, hafnium nitride oxide, or hafnium nitride.

For example, the insulator 130 preferably has a stacked-layer structureusing a material with high dielectric strength such as siliconoxynitride and a high dielectric constant (high-k) material. In thecapacitor 100 having such a structure, a sufficient capacitance can beprovided owing to the high dielectric constant (high-k) insulator, andthe dielectric strength can be increased owing to the insulator withhigh dielectric strength, so that the electrostatic breakdown of thecapacitor 100 can be prevented.

Examples of the insulator with a high dielectric constant (high-k)material (a material having a high relative permittivity) includegallium oxide, hafnium oxide, zirconium oxide, an oxide containingaluminum and hafnium, an oxynitride containing aluminum and hafnium, anoxide containing silicon and hafnium, an oxynitride containing siliconand hafnium, and a nitride containing silicon and hafnium.

As the material having a high dielectric strength (a material having alow relative permittivity), silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, silicon oxide to which fluorine isadded, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, porous silicon oxide, a resin, or thelike can be given.

<Wiring Layers>

Wiring layers provided with an interlayer film, a wiring, a plug, andthe like may be provided between the structure bodies. A plurality ofwiring layers can be provided in accordance with the design. Note that aplurality of conductors functioning as plugs or wirings are collectivelydenoted by the same reference numeral in some cases. Furthermore, inthis specification and the like, a wiring and a plug electricallyconnected to the wiring may be a single component. That is, there are acase where part of a conductor functions as a wiring and a case wherepart of a conductor functions as a plug.

For example, an insulator 320, an insulator 322, an insulator 324, andan insulator 326 are stacked over the transistor 300 in this order asinterlayer films. A conductor 328, a conductor 330, and the like thatare electrically connected to the capacitor 100 or the transistor 200are embedded in the insulator 320, the insulator 322, the insulator 324,and the insulator 326. Note that the conductor 328 and the conductor 330function as plugs or wirings.

The insulator functioning as an interlayer film may function as aplanarization film that covers an uneven shape thereunder. For example,a top surface of the insulator 322 may be planarized by planarizationtreatment using a chemical mechanical polishing (CMP) method or the liketo improve planarity.

A wiring layer may be provided over the insulator 326 and the conductor330. For example, in FIG. 21 , an insulator 350, an insulator 352, andan insulator 354 are stacked sequentially. Furthermore, a conductor 356is formed in the insulator 350, the insulator 352, and the insulator354. The conductor 356 functions as a plug or a wiring.

Similarly, a conductor 218, a conductor (the conductor 205) included inthe transistor 200, and the like are embedded in the insulator 210, theinsulator 212, the insulator 214, and the insulator 216. Note that theconductor 218 has a function of a plug or a wiring that is electricallyconnected to the capacitor 100 or the transistor 300. In addition, theinsulator 150 is provided over the conductor 120 and the insulator 130.

Examples of an insulator that can be used as an interlayer film includean insulating oxide, an insulating nitride, an insulating oxynitride, aninsulating nitride oxide, an insulating metal oxide, an insulating metaloxynitride, and an insulating metal nitride oxide.

For example, when a material having a low relative permittivity is usedfor the insulator functioning as an interlayer film, the parasiticcapacitance generated between wirings can be reduced. Thus, a materialis preferably selected depending on the function of an insulator.

For example, as the insulator 150, the insulator 212, the insulator 352,the insulator 354, and the like, an insulator having a low relativepermittivity is preferably used. For example, the insulators eachpreferably include silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, silicon oxide to which fluorine is added,silicon oxide to which carbon is added, silicon oxide to which carbonand nitrogen are added, porous silicon oxide, a resin, or the like.Alternatively, the insulators each preferably have a stacked-layerstructure of a resin and silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, silicon oxide to which fluorine isadded, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, or porous silicon oxide. When siliconoxide or silicon oxynitride, which is thermally stable, is combined witha resin, the stacked-layer structure can have thermal stability and alow relative permittivity. Examples of the resin include polyester,polyolefin, polyamide (e.g., nylon and aramid), polyimide,polycarbonate, and acrylic

When the transistor using an oxide semiconductor is surrounded by aninsulator that has a function of inhibiting the transmission of oxygenand impurities such as hydrogen, the electrical characteristics of thetransistor can be stable. Thus, an insulator having a function ofinhibiting the transmission of oxygen and impurities such as hydrogen ispreferably used for the insulator 210, the insulator 350, and the like.

As an insulator having a function of inhibiting the transmission ofoxygen and impurities such as hydrogen, a single layer or a stackedlayer of an insulator containing, for example, boron, carbon, nitrogen,oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine,argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium,hafnium, or tantalum is used. Specifically, for the insulator having afunction of inhibiting the transmission of oxygen and impurities such ashydrogen, a metal oxide such as aluminum oxide, magnesium oxide, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide; silicon nitrideoxide; silicon nitride; or the like can be used.

For the conductors that can be used as a wiring or a plug, a materialcontaining one or more kinds of metal elements selected from aluminum,chromium, copper, silver, gold, platinum, tantalum, nickel, titanium,molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium,zirconium, beryllium, indium, ruthenium, and the like can be used.Furthermore, a semiconductor having high electrical conductivity,typified by polycrystalline silicon containing an impurity element suchas phosphorus, or silicide such as nickel silicide may be used.

For example, for the conductor 328, the conductor 330, the conductor356, the conductor 218, the conductor 112, and the like, a single layeror a stacked layer of a conductive material such as a metal material, analloy material, a metal nitride material, or a metal oxide materialwhich is formed using the above materials can be used. It is preferableto use a high-melting-point material that has both heat resistance andconductivity, such as tungsten or molybdenum, and it is particularlypreferable to use tungsten. Alternatively, a low-resistance conductivematerial such as aluminum or copper is preferably used. The use of alow-resistance conductive material can reduce wiring resistance.

<<Wirings or Plugs in a Layer Provided with an Oxide Semiconductor>>

In the case where an oxide semiconductor is used in the transistor 200,an insulator including an excess oxygen region is provided in thevicinity of the oxide semiconductor in some cases. In that case, aninsulator having a barrier property is preferably provided between theinsulator including the excess oxygen region and the conductor providedin the insulator including the excess oxygen region.

For example, an insulator 276 is preferably provided between theinsulator 224 and the conductor 246 in FIG. 21 . In particular, theconductor 246 is preferably provided in contact with the insulator 224containing an excess oxygen region, and the insulator 222, the insulator254, and the insulator 244 which sandwich the insulator 224. Since theinsulator 276 is provided in contact with the insulator 222 and theinsulator 281, the insulator 224 and the transistor 200 can be sealed bythe insulators having a barrier property. It is also preferable that theinsulator 276 be in contact with part of the insulator 280. When theinsulator 276 extends to the insulator 280, diffusion of oxygen andimpurities can be further inhibited.

That is, when the insulator 276 is provided, absorption of excess oxygencontained in the insulator 224 by the conductor 246 can be inhibited. Inaddition, when the insulator 276 is included, diffusion of hydrogen,which is an impurity, into the transistor 200 through the conductor 246can be inhibited.

For the insulator 276, an insulating material having a function ofinhibiting diffusion of oxygen and impurities such as water and hydrogenis preferably used. For example, aluminum oxide or hafnium oxide ispreferably used. Alternatively, for example, a metal oxide such asmagnesium oxide, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide;silicon nitride oxide; or silicon nitride can be used.

The above is the description of the structure example. With the use ofthe structure, a change in electrical characteristics can be reduced andreliability can be improved in a semiconductor device using a transistorincluding an oxide semiconductor. Alternatively, a transistor includingan oxide semiconductor with a high on-state current can be provided.Alternatively, a transistor including an oxide semiconductor with lowoff-state current can be provided. Alternatively, a semiconductor devicewith low power consumption can be provided.

[Memory Device 2]

FIG. 22 illustrates an example of a memory device using thesemiconductor device of one embodiment of the present invention. Thememory device illustrated in FIG. 22 includes a transistor 400 inaddition to the semiconductor device including the transistor 200, thetransistor 300, and the capacitor 100 illustrated in FIG. 21 .

The transistor 400 can control a second gate voltage of the transistor200. For example, a first gate and a second gate of the transistor 400are diode-connected to a source of the transistor 400, and the sourcethereof is connected to the second gate of the transistor 200. When anegative potential of the second gate of the transistor 200 is retainedin this structure, a first gate-source voltage and a second gate-sourcevoltage of the transistor 400 are 0 V. In the transistor 400, a draincurrent when the second gate voltage and the first gate voltage are 0 Vis extremely low; thus, the negative potential of the second gate of thetransistor 200 can be held for a long time even without power supply tothe transistor 200 and the transistor 400. Accordingly, the memorydevice including the transistor 200 and the transistor 400 can retainstored data for a long time.

In FIG. 22 , the wiring 1001 is electrically connected to the source ofthe transistor 300. The wiring 1002 is electrically connected to thedrain of the transistor 300. The wiring 1003 is electrically connectedto one of the source and the drain of the transistor 200. The wiring1004 is electrically connected to the gate of the transistor 200. Thewiring 1006 is electrically connected to a back gate of the transistor200. The gate of the transistor 300 and the other of the source and thedrain of the transistor 200 are electrically connected to one electrodeof the capacitor 100. The wiring 1005 is electrically connected to theother electrode of the capacitor 100. A wiring 1007 is electricallyconnected to the source of the transistor 400. A wiring 1008 iselectrically connected to a gate of the transistor 400. A wiring 1009 iselectrically connected to a back gate of the transistor 400. A wiring1010 is electrically connected to the drain of the transistor 400. Thewiring 1006, the wiring 1007, the wiring 1008, and the wiring 1009 areelectrically connected to each other.

When the memory devices illustrated in FIG. 22 are arranged in a matrixlike the memory devices illustrated in FIG. 21 , a memory cell array canbe formed. Note that one transistor 400 can control second gate voltagesof the transistors 200. For this reason, the number of providedtransistors 400 is preferably smaller than the number of transistors200.

[Transistor 400]

The transistor 400 and the transistors 200 are formed in the same layerand thus can be fabricated in parallel. The transistor 400 includes aconductor 460 (a conductor 460 a and a conductor 460 b) functioning as afirst gate electrode; a conductor 405 (a conductor 405 a and a conductor405 b) functioning as a second gate electrode; the insulator 222, theinsulator 224, and an insulator 450 each functioning as a gateinsulating layer; an oxide 430 c including a region where a channel isformed; a conductor 442 a functioning as one of a source and a drain; anoxide 431 a and an oxide 431 b; a conductor 442 b functioning as theother of the source and the drain; an oxide 432 a and an oxide 432 b;and a conductor 440 (a conductor 440 a and a conductor 440 b).

In the transistor 400, the conductor 405 is in the same layer as theconductor 205. The oxide 431 a and the oxide 432 a are in the same layeras the oxide 230 a, and the oxide 431 b and the oxide 432 b are in thesame layer as the oxide 230 b. The conductor 442 is in the same layer asthe conductor 242. The oxide 430 c is in the same layer as the oxide 230c. The insulator 450 is in the same layer as the insulator 250. Theconductor 460 is in the same layer as the conductor 260.

Note that the structure bodies formed in the same layer can be formed atthe same time. For example, the oxide 430 c can be formed by processingan oxide film to be the oxide 230 c.

In the oxide 430 c functioning as an active layer of the transistor 400,oxygen vacancies and impurities such as hydrogen and water are reduced,as in the oxide 230 or the like. Accordingly, the threshold voltage ofthe transistor 400 can be higher than 0 V, an off-state current can bereduced, and the drain current when the second gate voltage and thefirst gate voltage are 0 V can be extremely low.

<<Dicing Line>>

A dicing line (also referred to as a scribe line, a dividing line, or acutting line in some cases) that is provided when a large-sizedsubstrate is divided into semiconductor elements so that a plurality ofsemiconductor devices are each formed in a chip form will be describedbelow. Examples of a dividing method include the case where a groove (adicing line) for dividing the semiconductor elements is formed on thesubstrate, and then the substrate is cut along the dicing line to divide(split) it into a plurality of semiconductor devices.

Here, for example, as illustrated in FIG. 22 , it is preferable toperform design so that a region in which the insulator 254 and theinsulator 222 are in contact with each other is the dicing line. Thatis, an opening is provided in the insulator 224 in the vicinity of theregion to be the dicing line that is provided in an outer edge of thetransistor 400 and the memory cell including a plurality of transistors200. The insulator 254 and the insulator 244 are provided to cover theside surface of the insulator 224.

That is, in the opening provided in the insulator 224, the insulator 222is in contact with the insulator 254. For example, the insulator 222 andthe insulator 254 may be formed using the same material and the samemethod. When the insulator 222 and the insulator 254 are formed usingthe same material and the same method, the adhesion therebetween can beincreased. For example, aluminum oxide is preferably used.

With such a structure, the insulator 224, the transistor 200 and thetransistor 400 can be enclosed with the insulator 222 and the insulator254. Since the insulator 222 and the insulator 254 have a function ofinhibiting diffusion of oxygen, hydrogen, and water even when thesubstrate is divided into circuit regions each of which is provided withthe semiconductor elements in this embodiment to form a plurality ofchips, the entry and diffusion of impurities such as hydrogen and waterfrom the direction of a side surface of the divided substrate to thetransistor 200 and the transistor 400 can be inhibited.

Furthermore, in the structure, excess oxygen in the insulator 224 can beinhibited from being diffused into the outside of the insulator 254 andthe insulator 222. Accordingly, excess oxygen in the insulator 224 isefficiently supplied to the oxide where the channel is formed in thetransistor 200 or the transistor 400. The oxygen can reduce oxygenvacancies in the oxide where the channel is formed in the transistor 200or the transistor 400. Thus, the oxide where the channel is formed inthe transistor 200 or the transistor 400 can be an oxide semiconductorwith a low density of defect states and stable characteristics. That is,a change in electrical characteristics of the transistors 200 or thetransistor 400 can be inhibited and reliability can be improved.

This embodiment can be implemented in an appropriate combination withthe structures described in the other embodiments and the like.

Embodiment 4

In this embodiment, a memory device of one embodiment of the presentinvention including a transistor in which an oxide is used for asemiconductor (hereinafter referred to as an OS transistor in somecases) and a capacitor (hereinafter, such a memory device is alsoreferred to as an OS memory device in some cases), will be describedwith reference to FIG. 23 and FIG. 24 . The OS memory device includes atleast a capacitor and an OS transistor that controls the charging anddischarging of the capacitor. Since the OS transistor has an extremelylow off-state current, the OS memory device has excellent retentioncharacteristics and thus can function as a nonvolatile memory.

<Structure Example of Memory Device>

FIG. 23A illustrates a structure example of the OS memory device. Amemory device 1400 includes a peripheral circuit 1411 and a memory cellarray 1470. The peripheral circuit 1411 includes a row circuit 1420, acolumn circuit 1430, an output circuit 1440, and a control logic circuit1460.

The column circuit 1430 includes, for example, a column decoder, aprecharge circuit, a sense amplifier, a write circuit, and the like. Theprecharge circuit has a function of precharging wirings. The senseamplifier has a function of amplifying a data signal read from a memorycell. Note that the wirings are connected to the memory cell included inthe memory cell array 1470, and will be described later in detail. Theamplified data signal is output as a data signal RDATA to the outside ofthe memory device 1400 through the output circuit 1440. The row circuit1420 includes, for example, a row decoder and a word line drivercircuit, and can select a row to be accessed.

As power supply voltages from the outside, a low power supply voltage(VSS), a high power supply voltage (VDD) for the peripheral circuit1411, and a high power supply voltage (VIL) for the memory cell array1470 are supplied to the memory device 1400. Control signals (CE, WE,and RE), an address signal ADDR, and a data signal WDATA are also inputto the memory device 1400 from the outside. The address signal ADDR isinput to the row decoder and the column decoder, and the WDATA is inputto the write circuit.

The control logic circuit 1460 processes the signals (CE, WE, and RE)input from the outside, and generates control signals for the rowdecoder and the column decoder. The CE is a chip enable signal, the WEis a write enable signal, and the RE is a read enable signal. Signalsprocessed by the control logic circuit 1460 are not limited thereto, andother control signals may be input as necessary.

The memory cell array 1470 includes a plurality of memory cells MC and aplurality of wirings arranged in a matrix. Note that the number of thewirings that connect the memory cell array 1470 to the row circuit 1420depends on the structure of the memory cell MC, the number of the memorycells MC in a column, and the like. The number of the wirings thatconnect the memory cell array 1470 to the column circuit 1430 depends onthe structure of the memory cell MC, the number of the memory cells MCin a row, and the like.

Note that FIG. 23(A) illustrates an example in which the peripheralcircuit 1411 and the memory cell array 1470 are formed on the sameplane; however, this embodiment is not limited thereto. For example, asillustrated in FIG. 23(B), the memory cell array 1470 may be providedover the peripheral circuit 1411 to partly overlap with the peripheralcircuit 1411. For example, the sense amplifier may be provided below thememory cell array 1470 so that they overlap with each other.

FIG. 24 illustrate structure examples of a memory cell applicable to thememory cell MC.

[DOSRAM]

FIGS. 24(A) to 24(C) each illustrate a circuit structure example of amemory cell of a DRAM. In this specification and the like, a DRAM usinga memory cell including one OS transistor and one capacitor is referredto as DOSRAM (Dynamic Oxide Semiconductor Random Access Memory) in somecases. A memory cell 1471 illustrated in FIG. 24(A) includes atransistor M1 and a capacitor CA. Note that the transistor M1 includes agate (also referred to as a front gate in some cases) and a back gate.

A first terminal of the transistor M1 is connected to a first terminalof the capacitor CA. A second terminal of the transistor M1 is connectedto a wiring BIL. The gate of the transistor M1 is connected to a wiringWOL. The back gate of the transistor M1 is connected to a wiring BGL. Asecond terminal of the capacitor CA is connected to a wiring CAL.

The wiring BIL functions as a bit line, and the wiring WOL functions asa word line. The wiring CAL functions as a wiring for applying apredetermined potential to the second terminal of the capacitor CA. Inthe time of data writing and data reading, a low-level potential ispreferably applied to the wiring CAL. The wiring BGL functions as awiring for applying a potential to the back gate of the transistor M1.By applying a given potential to the wiring BGL, the threshold voltageof the transistor M1 can be increased or decreased.

The memory cell MC is not limited to the memory cell 1471, and thecircuit structure can be changed. For example, as in a memory cell 1472illustrated in FIG. 24(B), the back gate of the transistor M1 may beconnected not to the wiring BGL but to the wiring WOL in the memory cellMC. Alternatively, for example, the memory cell MC may be a memory cellincluding a single-gate transistor, that is, the transistor M1 notincluding a back gate, as in a memory cell 1473 illustrated in FIG.24(C).

In the case where the semiconductor device described in any of the aboveembodiments is used in the memory cell 1471 and the like, the transistor200 can be used as the transistor M1, and the capacitor 100 can be usedas the capacitor CA. When an OS transistor is used as the transistor M1,the leakage current of the transistor M1 can be extremely low. That is,with the use of the transistor M1, written data can be retained for along time, and thus the frequency of the refresh operation for thememory cell can be decreased. In addition, refresh operation of thememory cell can be unnecessary. In addition, since the transistor M1 hasan extremely low leakage current, multi-level data or analog data can beretained in the memory cell 1471, the memory cell 1472, and the memorycell 1473.

In the DOSRAM, when the sense amplifier is provided below the memorycell array 1470 so that they overlap with each other as described above,the bit line can be shortened. Thus, the bit line capacitance can besmall, and the storage capacitance of the memory cell can be reduced.

[NOSRAM]

FIGS. 24(D) to 24(H) each illustrate a circuit structure example of again-cell memory cell including two transistors and one capacitor. Amemory cell 1474 illustrated in FIG. 24(D) includes a transistor M2, atransistor M3, and a capacitor CB. Note that the transistor M2 includesa front gate (simply referred to as a gate in some cases) and a backgate. In this specification and the like, a memory device including again-cell memory cell using an OS transistor as the transistor M2 isreferred to as NOSRAM (Nonvolatile Oxide Semiconductor RAM) in somecases.

A first terminal of the transistor M2 is connected to a first terminalof the capacitor CB. A second terminal of the transistor M2 is connectedto a wiring WBL. A gate of the transistor M2 is connected to the wiringWOL. Aback gate of the transistor M2 is connected to the wiring BGL. Asecond terminal of the capacitor CB is connected to the wiring CAL. Afirst terminal of the transistor M3 is connected to a wiring RBL. Asecond terminal of the transistor M3 is connected to a wiring SL. A gateof the transistor M3 is connected to the first terminal of the capacitorCB.

The wiring WBL functions as a write bit line, the wiring RBL functionsas a read bit line, and the wiring WOL functions as a word line. Thewiring CAL functions as a wiring for applying a predetermined potentialto the second terminal of the capacitor CB. In the time of data writing,data retaining, and data reading, a low-level potential is preferablyapplied to the wiring CAL. The wiring BGL functions as a wiring forapplying a potential to the back gate of the transistor M2. By applyinga given potential to the wiring BGL, the threshold voltage of thetransistor M2 can be increased or decreased.

The memory cell MC is not limited to the memory cell 1474, and thecircuit structure can be changed as appropriate. For example, as in amemory cell 1475 illustrated in FIG. 24(E), the back gate of thetransistor M2 may be connected not to the wiring BGL but to the wiringWOL in the memory cell MC. Alternatively, for example, the memory cellMC may be a memory cell including as single-gate transistor, that is,the transistor M2 not including a back gate, as in a memory cell 1476illustrated in FIG. 24(F). Alternatively, for example, in the memorycell MC, the wiring WBL and the wiring RBL may be combined into onewiring BIL, as in a memory cell 1477 illustrated in FIG. 24(G).

In the case where the semiconductor device described in any of the aboveembodiments is used in the memory cell 1474 and the like, the transistor200 can be used as the transistor M2, the transistor 300 can be used asthe transistor M3, and the capacitor 100 can be used as the capacitorCB. When an OS transistor is used as the transistor M2, the leakagecurrent of the transistor M2 can be extremely low. That is, with the useof the transistor M2, written data can be retained for a long time, andthus the frequency of the refresh operation for the memory cell can bedecreased. In addition, refresh operation of the memory cell can beunnecessary. In addition, since the transistor M2 has an extremely lowleakage current, multi-level data or analog data can be retained in thememory cell 1474. The same applies to the memory cells 1475 to 1477.

Note that the transistor M3 may be a transistor containing silicon in achannel formation region (hereinafter, also referred to as a Sitransistor in some cases). The conductivity type of the Si transistormay be of either an n-channel type or a p-channel type. The Sitransistor has higher field-effect mobility than the OS transistor insome cases. Therefore, a Si transistor may be used as the transistor M3functioning as a reading transistor. Furthermore, the transistor M2 canbe provided to be stacked over the transistor M3 when a Si transistor isused as the transistor M3; therefore, the area occupied by the memorycell can be reduced, leading to high integration of the memory device.

Alternatively, the transistor M3 may be an OS transistor. When an OStransistor is used as each of the transistors M2 and M3, the circuit ofthe memory cell array 1470 can be formed using only n-channeltransistors.

FIG. 24(H) illustrates an example of a gain-cell memory cell includingthree transistors and one capacitor. A memory cell 1478 illustrated inFIG. 24(H) includes transistors M4 to M6 and a capacitor CC. Thecapacitor CC is provided as appropriate. The memory cell 1478 iselectrically connected to the wiring BIL, a wiring RWL, a wiring WWL,the wiring BGL, and a wiring GNDL. The wiring GNDL is a wiring forsupplying a low-level potential. Note that the memory cell 1478 may beelectrically connected to the wirings RBL and WBL instead of the wiringBIL.

The transistor M4 is an OS transistor including a back gate that iselectrically connected to the wiring BGL. Note that the back gate andthe gate of the transistor M4 may be electrically connected to eachother. Alternatively, the transistor M4 may not include the back gate.

Note that each of the transistors M5 and M6 may be an n-channel Sitransistor or a p-channel Si transistor. Alternatively, the transistorsM4 to M6 may be OS transistors, in which case the circuit of the memorycell array 1470 can be formed using only n-channel transistors.

In the case where the semiconductor device described in any of the aboveembodiments is used in the memory cell 1478, the transistor 200 can beused as the transistor M4, the transistor 300 can be used as thetransistors M5 and M6, and the capacitor 100 can be used as thecapacitor CC. When an OS transistor is used as the transistor M4, theleakage current of the transistor M4 can be extremely low.

Note that the structures of the peripheral circuit 1411, the memory cellarray 1470, and the like described in this embodiment are not limited tothe above. Positions and functions of these circuits, wirings connectedto the circuits, circuit elements, and the like can be changed, deleted,or added as needed.

The structure described in this embodiment can be used in an appropriatecombination with the structures described in the other embodiments andthe like.

Embodiment 5

In this embodiment, an example of a chip 1200 on which the semiconductordevice of the present invention is mounted will be described withreference to FIG. 25 . A plurality of circuits (systems) are mounted onthe chip 1200. The technique for integrating a plurality of circuits(systems) on one chip as described above is referred to as system onchip (SoC) in some cases.

As illustrated in FIG. 25(A), the chip 1200 includes a CPU (CentralProcessing Unit) 1211, a GPU (Graphics Processing Unit) 1212, one ormore of analog arithmetic units 1213, one or more of memory controllers1214, one or more of interfaces 1215, one or more of network circuits1216, and the like.

A bump (not illustrated) is provided on the chip 1200, and asillustrated in FIG. 25(B), the chip 1200 is connected to a first surfaceof a printed circuit board (PCB) 1201. A plurality of bumps 1202 areprovided on the rear side of the first surface of the PCB 1201, and thePCB 1201 is connected to a motherboard 1203.

A memory device such as a DRAM 1221 or a flash memory 1222 may beprovided over the motherboard 1203. For example, the DOSRAM described inthe above embodiment can be used as the DRAM 1221. For example, theNOSRAM described in the above embodiment can be used as the flash memory1222.

The CPU 1211 preferably includes a plurality of CPU cores. Furthermore,the GPU 1212 preferably includes a plurality of GPU cores. The CPU 1211and the GPU 1212 may each include a memory for storing data temporarily.Alternatively, a common memory for the CPU 1211 and the GPU 1212 may beprovided in the chip 1200. The NOSRAM or the DOSRAM described above canbe used as the memory. The GPU 1212 is suitable for parallel computationof a number of data and thus can be used for image processing orproduct-sum operation. When an image processing circuit or a product-sumoperation circuit including an oxide semiconductor of the presentinvention is provided in the GPU 1212, image processing and product-sumoperation can be performed with low power consumption.

In addition, since the CPU 1211 and the GPU 1212 are provided in thesame chip, a wiring between the CPU 1211 and the GPU 1212 can beshortened; accordingly, the data transfer from the CPU 1211 to the GPU1212, the data transfer between the memories included in the CPU 1211and the GPU 1212, and the transfer of arithmetic operation results fromthe GPU 1212 to the CPU 1211 after the arithmetic operation in the GPU1212 can be performed at high speed.

The analog arithmetic unit 1213 includes one or both of an A/D(analog/digital) converter circuit and a D/A (digital/analog) convertercircuit. Furthermore, the analog arithmetic unit 1213 may include theabove-described product-sum operation circuit.

The memory controller 1214 includes a circuit functioning as acontroller of the DRAM 1221 and a circuit functioning as the interfaceof the flash memory 1222.

The interface 1215 includes an interface circuit for an externalconnection device such as a display device, a speaker, a microphone, acamera, or a controller. Examples of the controller include a mouse, akeyboard, and a game controller. As such an interface, USB (UniversalSerial Bus), HDMI (registered trademark) (High-Definition MultimediaInterface), or the like can be used.

The network circuit 1216 includes a network circuit such as a LAN (LocalArea Network). Furthermore, the network circuit 1216 may include acircuit for network security.

The circuits (systems) can be formed in the chip 1200 in the samemanufacturing process. Therefore, even when the number of circuitsneeded for the chip 1200 is increased, there is no need to increase thenumber of steps in the manufacturing process; thus, the chip 1200 can bemanufactured at low cost.

The motherboard 1203 provided with the PCB 1201 on which the chip 1200including the GPU 1212 is mounted, the DRAM 1221, and the flash memory1222 can be referred to as a GPU module 1204.

The GPU module 1204 includes the chip 1200 formed using the SoCtechnology, and thus can have a small size. Furthermore, the GPU module1204 is excellent in image processing, and thus is suitably used in aportable electronic device such as a smartphone, a tablet terminal, alaptop PC, or a portable (mobile) game console. Furthermore, theproduct-sum operation circuit using the GPU 1212 can implement thearithmetic operation in a deep neural network (DNN), a convolutionalneural network (CNN), a recurrent neural network (RNN), an autoencorder,a deep Boltzmann machine (DBM), a deep belief network (DBN), or thelike; thus, the chip 1200 can be used as an AI chip or the GPU module1204 can be used as an AI system module.

The structure described in this embodiment can be used in an appropriatecombination with the structures described in the other embodiments.

Embodiment 6

In this embodiment, application examples of the memory device using thesemiconductor device described in the above embodiment will bedescribed. The semiconductor device described in the above embodimentcan be applied to, for example, memory devices of a variety ofelectronic devices (e.g., information terminals, computers, smartphones,e-book readers, digital cameras (including video cameras), videorecording/reproducing devices, and navigation systems). Here, thecomputers refer not only to tablet computers, notebook computers, anddesktop computers, but also to large computers such as server systems.Alternatively, the semiconductor device described in the aboveembodiment is applied to removable memory devices such as memory cards(e.g., SD cards), USB memories, and SSDs (solid state drives). FIG. 26schematically illustrates some structure examples of removable memorydevices. The semiconductor device described in the above embodiment isprocessed into a packaged memory chip and used in a variety of storagedevices and removable memories, for example.

FIG. 26(A) is a schematic view of a USB memory. A USB memory 1100includes a housing 1101, a cap 1102, a USB connector 1103, and asubstrate 1104. The substrate 1104 is held in the housing 1101. Forexample, a memory chip 1105 and a controller chip 1106 are attached tothe substrate 1104. The semiconductor device described in the aboveembodiment can be incorporated in the memory chip 1105 or the like onthe substrate 1104.

FIG. 26(B) is a schematic external view of an SD card, and FIG. 26(C) isa schematic view of the internal structure of the SD card. An SD card1110 includes a housing 1111, a connector 1112, and a substrate 1113.The substrate 1113 is held in the housing 1111. For example, a memorychip 1114 and a controller chip 1115 are attached to the substrate 1113.When the memory chip 1114 is also provided on the rear surface side ofthe substrate 1113, the capacity of the SD card 1110 can be increased.In addition, a wireless chip with a radio communication function may beprovided on the substrate 1113. With this, data can be read from andwritten in the memory chip 1114 by radio communication between a hostdevice and the SD card 1110. The semiconductor device described in theabove embodiment can be incorporated in the memory chip 1114 or the likeon the substrate 1113.

FIG. 26(D) is a schematic external view of an SSD, and FIG. 26(E) is aschematic view of the internal structure of the SSD. An SSD 1150includes a housing 1151, a connector 1152, and a substrate 1153. Thesubstrate 1153 is held in the housing 1151. For example, a memory chip1154, a memory chip 1155, and a controller chip 1156 are attached to thesubstrate 1153. The memory chip 1155 is a work memory for the controllerchip 1156, and a DOSRAM chip may be used, for example. When the memorychip 1154 is also provided on the rear surface side of the substrate1153, the capacity of the SSD 1150 can be increased. The semiconductordevice described in the above embodiment can be incorporated in thememory chip 1154 or the like on the substrate 1153.

This embodiment can be implemented in an appropriate combination withthe structures described in the other embodiments and the like.

Embodiment 7

<Electronic Device>

The semiconductor device of one embodiment of the present invention canbe used for processors such as CPUs and GPUs or chips. FIG. 27illustrates specific examples of electronic devices including aprocessor such as a CPU or a GPU or a chip of one embodiment of thepresent invention.

<Electronic Device and System>

The GPU or the chip of one embodiment of the present invention can beincorporated into a variety of electronic devices. Examples ofelectronic devices include a digital camera, a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, and an audio reproducing device in addition toelectronic devices provided with a relatively large screen, such as atelevision device, a desktop or laptop personal computer, a monitor fora computer and the like, digital signage, and a large game machine likea pachinko machine. When the integrated circuit or the chip of oneembodiment of the present invention is provided in an electronic device,the electronic device can include artificial intelligence.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display a video, data, or the like on a displayportion. When the electronic device includes the antenna and a secondarybattery, the antenna may be used for contactless power transmission.

The electronic device of one embodiment of the present invention mayinclude a sensor (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, electric field, current,voltage, electric power, radioactive rays, flow rate, humidity,gradient, oscillation, a smell, or infrared rays).

The electronic device of one embodiment of the present invention canhave a variety of functions. For example, the electronic device can havea function of displaying a variety of data (a still image, a movingimage, a text image, and the like) on a display portion, a touch panelfunction, a function of displaying a calendar, date, time, and the like,a function of executing a variety of software (programs), a wirelesscommunication function, and a function of reading out a program or datastored in a recording medium. FIG. 27 illustrates examples of electronicdevices.

[Mobile Phone]

FIG. 27(A) illustrates a mobile phone (smartphone), which is a type ofinformation terminal. An information terminal 5500 includes a housing5510 and a display portion 5511. As input interfaces, a touch panel isprovided in the display portion 5511 and a button is provided in thehousing 5510.

The information terminal 5500 can execute an application utilizingartificial intelligence, with the use of the chip of one embodiment ofthe present invention. Examples of the application utilizing artificialintelligence include an application for interpreting a conversation anddisplaying its content on the display portion 5511; an application forrecognizing letters, figures, and the like input to the touch panel ofthe display portion 5511 by a user and displaying them on the displayportion 5511; and an application for biometric authentication usingfingerprints, voice prints, or the like.

[Information Terminal 1]

FIG. 27(B) illustrates a desktop information terminal 5300. The desktopinformation terminal 5300 includes a main body 5301 of the informationterminal, a display 5302, and a keyboard 5303.

Like the information terminal 5500 described above, the desktopinformation terminal 5300 can execute an application utilizingartificial intelligence, with the use of the chip of one embodiment ofthe present invention. Examples of the application utilizing artificialintelligence include design-support software, text correction software,and software for automatic menu generation. Furthermore, with the use ofthe desktop information terminal 5300, novel artificial intelligence canbe developed.

Note that in the above description, a smartphone and a desktopinformation terminal are shown as examples of the electronic devices inFIGS. 27(A) and 27(B); alternatively, the electronic device can be aninformation terminal other than a smartphone and a desktop informationterminal. Examples of information terminals other than a smartphone anda desktop information terminal include a PDA (Personal DigitalAssistant), a laptop information terminal, and a workstation.

[Household Appliance]

FIG. 27(C) illustrates an electric refrigerator-freezer 5800 as anexample of a household appliance. The electric refrigerator-freezer 5800includes a housing 5801, a refrigerator door 5802, a freezer door 5803,and the like.

When the chip of one embodiment of the present invention is used in theelectric refrigerator-freezer 5800, the electric refrigerator-freezer5800 including artificial intelligence can be obtained. Utilizing theartificial intelligence enables the electric refrigerator-freezer 5800to have a function of automatically making a menu based on foods storedin the electric refrigerator-freezer 5800 and food expiration dates, forexample, a function of automatically adjusting the temperature to beappropriate for the foods stored in the electric refrigerator-freezer5800, and the like.

Although the electric refrigerator-freezer is described here as anexample of a household appliance, other examples of a householdappliance include a vacuum cleaner, a microwave oven, an electric oven,a rice cooker, a water heater, an IH cooker, a water server, aheating-cooling combination appliance such as an air conditioner, awashing machine, a drying machine, and an audio visual appliance.

[Game Machine]

FIG. 27(D) illustrates a portable game machine 5200 as an example of agame machine. The portable game machine includes a housing 5201, adisplay portion 5202, a button 5203, and the like.

When the GPU or the chip of one embodiment of the present invention isused in the portable game machine 5200, the portable game machine 5200with low power consumption can be obtained. Moreover, heat generationfrom a circuit can be reduced owing to low power consumption; thus, theinfluence of heat generation on the circuit, the peripheral circuit, andthe module can be reduced.

Furthermore, when the GPU or the chip of one embodiment of the presentinvention is used in the portable game machine 5200, the portable gamemachine 5200 including artificial intelligence can be obtained.

In general, the progress of a game, the actions and words of gamecharacters, and expressions of a phenomenon and the like in the game areprogramed in the game; however, the use of artificial intelligence inthe portable game machine 5200 enables expressions not limited by thegame program. For example, questions posed by the player, the progressof the game, time, and actions and words of game characters can bechanged for various expressions.

When a game requiring a plurality of players is played on the portablegame machine 5200, the artificial intelligence can create a virtual gameplayer; thus, the game can be played alone with the game player createdby the artificial intelligence as an opponent.

Although the portable game machine is illustrated as an example of agame machine in FIG. 27(D), the game machine using the GPU or the chipof one embodiment of the present invention is not limited thereto.Examples of the game machine using the GPU or the chip of one embodimentof the present invention include a home stationary game machine, anarcade game machine installed in entertainment facilities (a gamecenter, an amusement park, and the like), and a throwing machine forbatting practice installed in sports facilities.

[Moving Vehicle]

The GPU or the chip of one embodiment of the present invention can beused in an automobile, which is a moving vehicle, and around a driver'sseat in the automobile.

FIG. 27 (E1) illustrates an automobile 5700 as an example of a movingvehicle, and FIG. 27 (E2) is a diagram illustrating the periphery of awindshield inside the automobile. FIG. 27 (E2) illustrates a displaypanel 5701, a display panel 5702, and a display panel 5703 that areattached to a dashboard and a display panel 5704 that is attached to apillar.

The display panel 5701 to the display panel 5703 can provide variouskinds of information such as a speedometer, a tachometer, a mileage, afuel meter, a gearshift indicator, and air-condition setting. Thecontent, layout, or the like of the display on the display panels can bechanged as appropriate to suit the user's preference, so that the designcan be improved. The display panel 5701 to the display panel 5703 canalso be used as lighting devices.

The display panel 5704 can compensate for the view obstructed by thepillar (a blind spot) by showing an image taken by an imaging device(not illustrated) provided for the automobile 5700. That is, displayingan image taken by the imaging device provided on the outside of theautomobile 5700 leads to compensation for the blind spot and enhancementof safety. In addition, showing an image for compensating for the areawhich a driver cannot see makes it possible for the driver to confirmsafety more easily and comfortably. The display panel 5704 can also beused as a lighting device.

Since the GPU or the chip of one embodiment of the present invention canbe used as a component of artificial intelligence, the chip can be usedin an automatic driving system of the automobile 5700, for example. Thechip can also be used for a system for navigation, risk prediction, orthe like. The display panel 5701 to the display panel 5704 may displayinformation regarding navigation information, risk prediction, and thelike.

Although an automobile is described above as an example of a movingvehicle, moving vehicles are not limited to an automobile. Examples ofmoving vehicles include a train, a monorail train, a ship, and a flyingobject (a helicopter, an unmanned aircraft (a drone), an airplane, and arocket), and these moving vehicles can include a system utilizingartificial intelligence when equipped with the chip of one embodiment ofthe present invention.

[Broadcasting System]

The GPU or the chip of one embodiment of the present invention can beused in a broadcasting system.

FIG. 27(F) schematically shows data transmission in a broadcastingsystem. Specifically, FIG. 27(F) shows a path in which a radio wave (abroadcasting signal) transmitted from a broadcast station 5680 isdelivered to a television receiver (TV) 5600 of each household. The TV5600 includes a receiving device (not illustrated), and the broadcastsignal received by an antenna 5650 is transmitted to the TV 5600 throughthe receiving device.

Although a UHF (Ultra High Frequency) antenna is illustrated as theantenna 5650 in FIG. 27(F), a BS/110° CS antenna, a CS antenna, or thelike can also be used as the antenna 5650.

A radio wave 5675A and a radio wave 5675B are broadcast signals forterrestrial broadcasting; a radio wave tower 5670 amplifies the receivedradio wave 5675A and transmits the radio wave 5675B. Each household canview terrestrial TV broadcasting on the TV 5600 by receiving the radiowave 5675B with the antenna 5650. Note that the broadcasting system isnot limited to the terrestrial broadcasting shown in FIG. 27(F) and maybe satellite broadcasting using an artificial satellite, databroadcasting using an optical line, or the like.

The above-described broadcasting system may utilize artificialintelligence by using the chip of one embodiment of the presentinvention. When the broadcast data is transmitted from the broadcaststation 5680 to the TV 5600 at home, the broadcast data is compressed byan encoder. When the antenna 5650 receives the compressed broadcastdata, the compressed broadcast data is decompressed by a decoder of thereceiving device in the TV 5600. With the use of artificialintelligence, for example, a display pattern included in an image to bedisplayed can be recognized in motion compensation prediction, which isone of the compressing methods for the encoder. In-frame predictionutilizing artificial intelligence, for instance, can also be performed.For another example, when the broadcast data with low resolution isreceived and displayed on the TV 5600 with high resolution, imageinterpolation such as upconversion can be performed in the broadcastdata decompression by the decoder.

The above-described broadcasting system utilizing artificialintelligence is suitable for ultra-high definition television (UHDTV:4K, 8K) broadcasting, which needs a large amount of broadcast data.

As an application of artificial intelligence in the TV 5600, a recordingdevice including artificial intelligence may be provided in the TV 5600,for example. With such a structure, the artificial intelligence in therecording device can learn the user's preference, so that TV programsthat suit the user's preference can be recorded automatically.

The electronic devices, the functions of the electronic devices,application examples of artificial intelligence, its effects, and thelike described in this embodiment can be combined as appropriate withthe description of another electronic device.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments and the like.

Example

In this example, a transistor having a structure similar to that of thetransistor 200 illustrated in FIG. 10 (hereinafter referred to asSample 1) was fabricated as the semiconductor device of one embodimentof the present invention. Observation results of the semiconductordevice with a scanning transmission electron microscope (STEM) will bedescribed.

First, a structure of Sample 1 will be described. As illustrated in FIG.10 , Sample 1 includes the insulator 214, the conductor 205, theinsulator 216, the insulator 222, the insulator 224, the oxide 230 a,the oxide 230 b, the oxide 230 c, the conductor 242 a, the conductor 242b, the insulator 250, the conductor 260 a, the conductor 260 b, theinsulator 254, the insulator 280, and the insulator 274.

As the insulator 214, 40-nm-thick aluminum oxide was used. As theconductor 205, 60-nm-thick tungsten was used. As the insulator 216,60-nm-thick silicon oxynitride was used. As the insulator 222,5-nm-thick aluminum oxide deposited by an ALD method was used. As theinsulator 224, 35-nm-thick silicon oxynitride was used.

As the oxide 230 a, 5-nm-thick In—Ga—Zn oxide deposited by a DCsputtering method was used. In the deposition of the oxide 230 a, atarget with In:Ga:Zn=1:3:4 [atomic ratio] was used; an oxygen gas at 45sccm was used as a deposition gas; the deposition pressure was 0.7 Pa(measured with Miniature Gauge MG-2 manufactured by CANON ANELVACORPORATION); the deposition power was 500 W; the substrate temperaturewas 200° C.; and the target-substrate distance was 60 mm.

As the oxide 230 b, 15-nm-thick In—Ga—Zn oxide deposited by a DCsputtering method was used. In the deposition of the oxide 230 b, atarget with In:Ga:Zn=4:2:4.1 [atomic ratio] was used; an argon gas at 30sccm and an oxygen gas at 15 sccm were used as a deposition gas; thedeposition pressure was 0.7 Pa (measured with Miniature Gauge MG-2manufactured by CANON ANELVA CORPORATION); the deposition power was 500W; the substrate temperature was 200° C.; and the target-substratedistance was 60 mm.

As each of the conductor 242 a and the conductor 242 b, 25-nm-thicktantalum nitride was used. As the insulator 254, a stacked film of5-nm-thick aluminum oxide deposited by a sputtering method and3-nm-thick aluminum oxide deposited thereover by an ALD method was used.As the insulator 280, silicon oxynitride was used.

The oxide 230 c is a stacked film. As a lower film of the oxide 230 c,5-nm-thick In—Ga—Zn oxide deposited by a DC sputtering method was used.In the deposition of the lower film of the oxide 230 c, a target withIn:Ga:Zn=4:2:4.1 [atomic ratio] was used; an oxygen gas at 45 sccm wasused as a deposition gas; the deposition pressure was 0.7 Pa (measuredwith Miniature Gauge MG-2 manufactured by CANON ANELVA CORPORATION); thedeposition power was 500 W; the substrate temperature was 200° C.; andthe target-substrate distance was 60 mm.

As an upper film of the oxide 230 c, 5-nm-thick In—Ga—Zn oxide depositedby a DC sputtering method was used. Note that in the deposition of theupper film of the oxide 230 c, a target with In:Ga:Zn=1:3:4 [atomicratio] was used, and the other deposition conditions were similar tothose for the oxide 230 a.

As the insulator 250, 10-nm-thick silicon oxynitride was used. As theconductor 260 a, 5-nm-thick titanium nitride was used. As the conductor260 b, tungsten was used. As the insulator 274, 40-nm-thick aluminumoxide was used.

Sample 1 having the above structure is a transistor having a channellength of 60 nm and a channel width of 60 nm. Note that Sample 1includes, in addition to the above components, the conductor 240 and thelike in the insulator 280 and includes an insulating film functioning asan interlayer film, a conductive film functioning as a wiring, and thelike over the insulator 274.

A cross-sectional TEM image of fabricated Sample 1 was taken with“H-9500” manufactured by Hitachi High-Technologies Corporation at anacceleration voltage of 300 kV. FIG. 28 shows results of thecross-sectional TEM image that was taken. FIG. 28 is a cross-sectionalTEM image of the channel formation region in the oxide 230 and thevicinity thereof in the channel width direction.

FIG. 29 shows enlarged cross-sectional TEM images of a region A to aregion E shown in FIG. 28 . Here, the region A includes the oxide 230 cin contact with the top surface of the oxide 230 b. The region Bincludes the oxide 230 c in contact with the end portion of the topsurface of the oxide 230 b. The region C includes the oxide 230 c incontact with the side surface of the oxide 230 b. The region D includesthe oxide 230 c in contact with the side surface of the insulator 224.The region E includes the oxide 230 c in contact with the top surface ofthe insulator 224.

In the regions shown in FIG. 28 and FIG. 29 , the oxide 230 c wasdeposited with an extremely small thickness of approximately 2 nm to 5nm. However, as shown in FIG. 29(A) to FIG. 29(E), in the oxide 230 c ineach of the regions, a layered CAAC-OS was formed. Here, arrowsillustrated in FIG. 29(A) to FIG. 29(E) indicate a directionsubstantially perpendicular to the film of the oxide 230 c, and thedirections of the arrows are substantially the same as the normaldirection of layered crystals of the oxide 230 c, that is, the c-axisdirection of the CAAC-OS. Thus, it is found that the CAAC-OS of theoxide 230 c is arranged along unevenness of the formation surface of theoxide 230 c.

Moreover, Sample 2 having a structure similar to that of Sample 1 wasfabricated, and a cross-sectional TEM image of Sample 2 in the channellength direction was taken. FIG. 30 shows results of the cross-sectionalTEM image that was taken. FIG. 30 is a cross-sectional TEM image of thechannel formation region in the oxide 230 and the vicinity thereof inthe channel length direction, and is an enlarged view corresponding tothe vicinity of the interface between the conductor 242 a and the oxide230 c illustrated in FIG. 3 .

As shown in FIG. 30 , a layered CAAC-OS is formed in a region F of theoxide 230 c in Sample 2. Furthermore, the layer of the CAAC-OS in theregion F is aligned in substantially parallel to the side surface of theconductor 242 a. In other words, the c-axis of the CAAC-OS in the regionF is aligned substantially perpendicularly to the side surface of theconductor 242 a. In a portion of the oxide 230 c parallel to the oxide230 b, the layer of the CAAC-OS is substantially parallel to the layerof the CAAC-OS of the oxide 230 b.

In addition, Sample 3 including the oxide 12 provided over thesubstrate; the conductor 14 a and the conductor 14 b provided over theoxide 12 to be separated from each other; and the oxide 13 provided overthe oxide 12 and between the conductor 14 a and the conductor 14 b wasfabricated, as in FIG. 1(B). Note that a silicon wafer provided with100-nm-thick thermal oxidation film is used as the substrate. Each ofthe oxide 12 and the oxide 13 is a stacked film of an In—Ga—Zn oxide.

As a lower film of the oxide 12, 5-nm-thick In—Ga—Zn oxide deposited bya DC sputtering method was used. In the deposition of the lower film ofthe oxide 12, a target with In:Ga:Zn=1:3:4 [atomic ratio] was used; anoxygen gas at 45 sccm was used as a deposition gas; the depositionpressure was 0.7 Pa (measured with Miniature Gauge MG-2 manufactured byCANON ANELVA CORPORATION); the deposition power was 500 W; the substratetemperature was 200° C.; and the target-substrate distance was 60 mm.

As an upper film of the oxide 12, 15-nm-thick In—Ga—Zn oxide depositedby a DC sputtering method was used. In the deposition of the upper filmof the oxide 12, a target with In:Ga:Zn=4:2:4.1 [atomic ratio] was used;an oxygen gas at 45 sccm was used as a deposition gas; the depositionpressure was 0.7 Pa (measured with Miniature Gauge MG-2 manufactured byCANON ANELVA CORPORATION); the deposition power was 500 W; the substratetemperature was 200° C.; and the target-substrate distance was 60 mm.

As each of the conductor 14 a and the conductor 14 b, 25-nm-thicktantalum nitride was used. Over the conductor 14 a and the conductor 14b, a stacked film of 5-nm-thick aluminum oxide deposited by a sputteringmethod and 3-nm-thick aluminum oxide deposited by an ALD methodthereover was provided. In addition, a silicon oxynitride film wasprovided over the stacked film of the aluminum oxide.

As a lower film of the oxide 13, 8-nm-thick In—Ga—Zn oxide deposited bya DC sputtering method was used. Note that in the deposition of thelower film of the oxide 13, a target with In:Ga:Zn=4:2:4.1 [atomicratio] was used, and the other deposition conditions were similar tothose for the upper film of the oxide 12.

As an upper film of the oxide 13, 8-nm-thick In—Ga—Zn oxide deposited bya DC sputtering method was used. Note that in the deposition of theupper film of the oxide 13, a target with In:Ga:Zn=1:3:4 [atomic ratio]was used, and the other deposition conditions were similar to those forthe lower film of the oxide 12.

A cross-sectional TEM image of fabricated Sample 3 was taken. FIG. 31(A)shows results of the cross-sectional TEM image that was taken. FIG.31(A) is a cross-sectional TEM image of the channel formation region inthe oxide 12 and the vicinity of thereof in the channel lengthdirection, and is an enlarged view corresponding to the vicinity of theinterface between the conductor 14 b and the oxide 13 illustrated inFIG. 1 .

As shown in FIG. 31(A), a layered CAAC-OS was formed in a region G and aregion H of the oxide 13 in Sample 3. In the region G of the oxide 13parallel to the oxide 12, the layer of the CAAC-OS was alignedsubstantially parallel to the layer of the CAAC-OS in the oxide 12. Incontrast, the layer of the CAAC-OS in the region H was alignedsubstantially parallel to the side surface of the conductor 14 b. Inother words, the c-axis of the CAAC-OS in the region H was alignedsubstantially perpendicularly to the side surface of the conductor 14 b.Note that in part of the region H, there was a region where the layer ofthe CAAC-OS was not substantially parallel to the side surface of theconductor 14 b.

Moreover, FIG. 31(B) shows a crystal orientation map made using thecross-sectional TEM image of Sample 3. The crystal orientation map shownin FIG. 31(B) was made in such a manner that FFT analysis was performedfor each FFT window with a diameter of 15 nm, and the degree ofcrystallinity and the slope of the c-axis in each pixel in thecross-sectional TEM image of Sample 3 were calculated. In thecrystalline region in FIG. 31(B), as the color is darker, the slope ofthe c-axis with respect to the substrate surface is closer to 90°, andas the color is brighter, the slope of the c-axis with respect to thesubstrate surface is closer to 0° or 180°. In other words, in thecrystalline region in FIG. 31(B), as the color is darker, the c-axisbecomes more substantially perpendicular with respect to the substratesurface, and as the color is brighter, the c-axis becomes moresubstantially parallel to the substrate surface.

In FIG. 31(B), the c-axis of the CAAC-OS was aligned substantiallyperpendicularly to the substrate surface in the region G of the oxide 13parallel to the oxide 12, as in FIG. 31(A). Similarly, the c-axis of theCAAC-OS in the region H was sloped with respect to the substrate surfaceby approximately 200 to 400 and was aligned substantiallyperpendicularly to the side surface of the conductor 14 b. Note that inpart of the region H, there was a region where the c-axis of the CAAC-OSwas not sloped by approximately 200 to 400 with respect to the substratesurface.

At least part of the structure, the method, and the like described inthis example can be implemented in appropriate combination with theembodiments described in this specification.

REFERENCE NUMERALS

-   -   10 a: transistor, 10 b: transistor, 10 c: transistor, 12: oxide,        12 a: layer, 12 b: c-axis, 13: oxide, 13 a: layer, 13 b: c-axis,        14 a: conductor, 14 b: conductor, 100: capacitor, 110:        conductor, 112: conductor, 120: conductor, 130: insulator, 150:        insulator, 200: transistor, 205: conductor, 210: insulator, 212:        insulator, 214: insulator, 216: insulator, 218: conductor, 222:        insulator, 224: insulator, 230: oxide, 230 a: oxide, 230A: oxide        film, 230 b: oxide, 230B: oxide film, 230 c: oxide, 230C: oxide        film, 240: conductor, 240 a: conductor, 240 b: conductor, 241:        insulator, 241 a: insulator, 241 b: insulator, 242: conductor,        242 a: conductor, 242A: conductive film, 242 b: conductor, 242B:        conductor layer, 243: conductor, 243 a: conductor, 243A:        conductor layer, 243 b: conductor, 244: insulator, 244A:        insulating film, 246: conductor, 250: insulator, 250A:        insulating film, 254: insulator, 254A: insulating film, 260:        conductor, 260 a: conductor, 260Aa: conductive film, 260Ab:        conductive film, 260 b: conductor, 262: dummy gate, 262A: dummy        gate layer, 263: opening, 264: opening, 264 a: opening, 264 b:        opening, 265: dummy film, 274: insulator, 276: insulator, 280:        insulator, 281: insulator, 300: transistor, 311: substrate, 313:        semiconductor region, 314 a: low-resistance region, 314 b:        low-resistance region, 315: insulator, 316: conductor, 320:        insulator, 322: insulator, 324: insulator, 326: insulator, 328:        conductor, 330: conductor, 350: insulator, 352 insulator, 354:        insulator, 356: conductor, 400: transistor, 405: conductor, 405        a: conductor, 405 b: conductor, 430 c: oxide, 431 a: oxide, 431        b: oxide, 432 a: oxide, 432 b: oxide, 440: conductor, 440 a:        conductor, 440 b: conductor, 442: conductor, 442 a: conductor,        442 b: conductor, 450: insulator, 460: conductor, 460 a:        conductor, 460 b: conductor, 1001: wiring, 1002: wiring, 1003:        wiring, 1004: wiring, 1005: wiring, 1006: wiring, 1007: wiring,        1008: wiring, 1009: wiring, 1010: wiring, 1100: USB memory,        1101: housing, 1102: cap, 1103: USB connector, 1104: substrate,        1105: memory chip, 1106: controller chip, 1110: SD card, 1111:        housing, 1112: connector, 1113: substrate, 1114: memory chip,        1115: controller chip, 1150: SSD, 1151: housing, 1152:        connector, 1153: substrate, 1154: memory chip, 1155: memory        chip, 1156: controller chip, 1200: chip, 1201: PCB, 1202: bump,        1203: motherboard, 1204: GPU module, 1211: CPU, 1212: GPU, 1213:        analog arithmetic unit, 1214: memory controller, 1215:        interface, 1216: network circuit, 1221: DRAM, 1222: flash        memory, 1400: memory device, 1411: peripheral circuit, 1420: row        circuit, 1430: column circuit, 1440: output circuit, 1460:        control logic circuit, 1470: memory cell array, 1471: memory        cell, 1472: memory cell, 1473: memory cell, 1474: memory cell,        1475: memory cell, 1476: memory cell, 1477: memory cell, 1478:        memory cell, 5200: portable game machine, 5201: housing, 5202:        display portion, 5203: button, 5300: desktop information        terminal, 5301: main body, 5302: display, 5303: keyboard, 5500:        information terminal, 5510: housing, 5511: display portion,        5600: TV, 5650: antenna, 5670: radio wave tower, 5675A: radio        wave, 5675B: radio wave, 5680: broadcast station, 5700:        automobile, 5701: display panel, 5702: display panel, 5703:        display panel, 5704: display panel, 5800: electric        refrigerator-freezer, 5801: housing, 5802: a refrigerator door,        5803: a freezer door

The invention claimed is:
 1. A semiconductor device comprising: a firstgate electrode over a substrate; a first gate insulating film over thefirst gate electrode; a first oxide semiconductor layer over the firstgate insulating film; a second oxide semiconductor layer over and incontact with the first oxide semiconductor layer; a source electrode anda drain electrode over the first oxide semiconductor layer; aninterlayer insulating film over the source electrode and the drainelectrode; a second gate insulating film in an opening of the interlayerinsulating film; and a second gate electrode over the second gateinsulating film and in the opening of the interlayer insulating film,wherein each of the first oxide semiconductor layer and the second oxidesemiconductor layer comprises In, Ga, and Zn, wherein the first oxidesemiconductor layer has crystallinity, wherein the second oxidesemiconductor layer has crystallinity and c-axis alignment, wherein thesecond oxide semiconductor layer comprises a first region in contactwith a surface of the first oxide semiconductor layer and a secondregion in contact with a surface of the one of the source electrode andthe drain electrode, and wherein a direction of a c-axis of the firstregion is different from a direction of a c-axis of the second region.2. The semiconductor device according to claim 1, wherein the firstregion is a region where the c-axis is aligned substantiallyperpendicularly to the surface of the first oxide semiconductor layer,and wherein the second region is a region where the c-axis is alignedsubstantially perpendicularly to the surface of the one of the sourceelectrode and the drain electrode.
 3. The semiconductor device accordingto claim 1, wherein an atomic ratio of Zn is higher than an atomic ratioof In and an atomic ratio of Ga in the second oxide semiconductor layer,and wherein the atomic ratio of Ga is higher than the atomic ratio of Inin the second oxide semiconductor layer.
 4. A semiconductor devicecomprising: a first oxide semiconductor layer over a substrate; a secondoxide semiconductor layer over and in contact with the first oxidesemiconductor layer; a source electrode and a drain electrode over thefirst oxide semiconductor layer; an interlayer insulating film over thesource electrode and the drain electrode; a first gate insulating filmin an opening of the interlayer insulating film; and a first gateelectrode over the first gate insulating film and in the opening of theinterlayer insulating film, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layer comprisesIn, Ga, and Zn, wherein the first oxide semiconductor layer hascrystallinity, wherein the second oxide semiconductor layer hascrystallinity and c-axis alignment, wherein the second oxidesemiconductor layer comprises a first region in contact with a surfaceof the first oxide semiconductor layer and a second region in contactwith a surface of the one of the source electrode and the drainelectrode, and wherein a direction of a c-axis of the first region isdifferent from a direction of a c-axis of the second region.
 5. Thesemiconductor device according to claim 4, wherein the first region is aregion where the c-axis is aligned substantially perpendicularly to thesurface of the first oxide semiconductor layer, and wherein the secondregion is a region where the c-axis is aligned substantiallyperpendicularly to the surface of the one of the source electrode andthe drain electrode.
 6. The semiconductor device according to claim 4,wherein an atomic ratio of Zn is higher than an atomic ratio of In andan atomic ratio of Ga in the second oxide semiconductor layer, andwherein the atomic ratio of Ga is higher than the atomic ratio of In inthe second oxide semiconductor layer.
 7. A semiconductor devicecomprising: a first oxide semiconductor layer over a substrate; a secondoxide semiconductor layer over and in contact with the first oxidesemiconductor layer; a source electrode and a drain electrode over thefirst oxide semiconductor layer; an interlayer insulating film over thesource electrode and the drain electrode; a first gate insulating filmin an opening of the interlayer insulating film; and a first gateelectrode over the first gate insulating film and in the opening of theinterlayer insulating film, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layer comprisesIn, wherein the first oxide semiconductor layer has crystallinity,wherein the second oxide semiconductor layer has crystallinity andc-axis alignment, wherein the second oxide semiconductor layer comprisesa first region in contact with a surface of the first oxidesemiconductor layer and a second region in contact with a surface of theone of the source electrode and the drain electrode, and wherein adirection of a c-axis of the first region is different from a directionof a c-axis of the second region.
 8. The semiconductor device accordingto claim 7, wherein the first region is a region where the c-axis isaligned substantially perpendicularly to the surface of the first oxidesemiconductor layer, and wherein the second region is a region where thec-axis is aligned substantially perpendicularly to the surface of theone of the source electrode and the drain electrode.